Your guitar has a very precise and intricate geometry.  Any changes to that geometry either make it harder to play or make it sound badly (such as string buzzing).

With a guitar made of solid carbon fiber, the geometry will always stay the same, regardless of temperature and humidity.  A guitar made of a thick piece of solid hardwood is also fairly stable, which is why electric guitars are somewhat resistant to damage from changes in humidity and temperature.  Extreme dryness or heat/cold may cause hairline cracks to the finish, but the electric guitar will stay playable.  Nobody I know worries about humidifying electric guitars.

In contrast, acoustic guitars are built of very thin pieces of wood, and in most cases the soundboard is not even made of hardwood (most are made of spruce).  None of the wood has any protection from humidity changes because the inside surfaces are raw, with no varnish or sealant of any kind, so the wood "breathes" whatever air it is exposed to.  Because of all this, the woods can expand or contract significantly with temperature and humidity changes.  Because different kinds of woods are used in the same guitar, some pieces expand at different rates than others. 

Such swelling or shrinking can result in significant damage to the guitar, such as:

• The varnish (or polyurethane or polyester) used to finish the outside surfaces can develop fissures and cracks
  
• The soundboard can become brittle and have less mass, resulting in less richness in the sound
  
• The soundboard can rise to a more convex shape (resulting in a high, hard-to-play action)
• The soundboard can lower to a less convex or flat shape (resulting in string buzz, or sometimes in complete unplayability)
• The soundboard can crack and split (usually at the seam in the center - VERY bad for the guitar!)
• The backboard and sides can also crack and separate
  
• The neck can shrink narrower, causing the frets to hang out the sides of the fretboard, which ruins the smooth feel

The ideal environment for your guitar is the same environment in which it was manufactured.  That way, the wood will stay the same thickness, density, and geometry it was when it was formed into a guitar.  For all quality guitar manufacturers, the environment in the wood curing/storage and manufacturing areas of the factory is controlled at approximately 70 degrees and 50% humidity.  That is why your guitar should always be stored in 60-80 degrees F and 40-60% humidity.

Temperature has always been reasonably easy to control.  Perhaps not always convenient, but there is never any guesswork.  The bottom line for temperature is this: Pretend your guitar is an infant child dressed in a "onesey" with exposed arms and legs.  Never expose your guitar to any temperature you would not expose your baby to.  For example, never leave your guitar in your car.  Not in the summer, and not in the winter.  A student once asked me if she could leave her guitar in the car in the winter if it were wrapped in a blanket.  The answer is no.  The guitar has no internal source of heat and within an hour will be the same temperature as the inside of the car, blanket or no blanket.

Humidity is equally important as temperature, if not even more important.  Link below to see a video of Bob Taylor of Taylor Guitars demonstrating the importance of humidity control.

LINK HERE to see the damage caused by low humidity

Optional:
LINK HERE to see how the guitar was repaired
LINK HERE to see the results of the repair

In the past, controlling humidity has been problematic and complicated.  First, you had to purchase a hydrometer (humidity gauge), or else you had to just guess what the humidity was.  If you determined or guessed that the humidity was too low, you had to insert a "guitar humidifier" consisting of a damp sponge in a plastic case into the guitar case and leave it in until the sponge dried out, then repeat, until the water content of the guitar wood was correct.  But you had to be very careful to not over-humidify the guitar with this process, because too much humidity is almost as bad for the guitar as is not enough humidity.  Even with a gauge, there was a lot of guesswork, trial and error, and worry.

All that has changed in the last few years, since D'Addario/Planet Waves introduced the world's first and only automatic humidity control for guitars: the Humidipak two-way humidification system (below).  It is available online and in guitar stores for $20.  This amazing invention controls the relative humidity in the guitar case at 45-55%, with no hassle, no worries, and no calculating or measuring with gauges.  If there is too much humidity, the Humidipak sucks the excess water vapor out of the air.  If there isn't enough humidity, it releases water vapor into the air.  Thus, your guitar is always exposed to the perfect environment whenever it is in its case.  The Humidipak system takes all the guesswork and hassle out of maintaining proper humidity whenever your guitar is not being played.  There are only two downsides of the Humidipak system.  First, it requires that you have a hardshell case.  It does not work in gig bags or canvas cases.  Second, there is an ongoing cost.  The gel/salt/water packets eventually dry out and have to be replaced, and until recently they were fairly pricey.  Fortunately, the price has of replacement packs has recently dropped to about $15.  The ease and peace of mind are worth it, especially if you have an over-$500 guitar.

I highly recommend that anyone who has an over-$1,500 guitar purchase and always use the Humidipak system.  For under-$500 guitars it isn't worth the ongoing cost.  Between $500 and $1,500, it's a toss up.  If you want more information, there are many videos on YouTube demonstrating and reviewing the Humidipak system. 

Whenever you take your guitar out if the case to play it, be sure to close the case tightly with the Humidipaks inside.  Otherwise the packs will start releasing their moisture into the room instead of your guitar, shortening their life.

When the gel packs turn stiff and hard (they feel like crystals rather than feeling like gel), they are no longer working and need to be replaced.  Some people on the web have reported success re-hydrating the packs, thus saving money and extending their life.  Buy an extra set of the packs.  When the packs you are using start to crystalize, take them out and put the other set in.  Don't throw away the aluminized Zip-Lock style pouches they came in!  Take the old packs, place them in a Tupperware or Rubbermaid container with a bowl of water in it to re-hydrate them.  Do not let the packs touch the water!  Either place the water in the bottom of the container and use a cup or something else to place the packs on above the water, or place the water in a bowl inside the container to the side of the packs.  In a few days the packs will be re-hydrated and can be placed back into the Zip-Lock pouch they came in, stored and ready for re-use.  In this manner you can probably get 3 or more cycles out of one set of packs.

An even better way to accomplish the same thing is to purchase an Oasis OH-1 guitar humidifier for about $20.  Fill it with distilled water and put it in the sound hole with the Humidipaks.  Refill it every two weeks.  The Humidipaks will absorb extra humidity during the first week, then release that extra humidity during the second week, after the Oasis dries out.  I have gotten years out of a set of Humidipaks instead of months, by using this method.

If you cannot afford the Humidipak system, you can still hydrate your guitar, just not as accurately.  In the Utah climate, anything is better than nothing.  For example, you can just use an Oasis OH-1 without the Humidipaks.  Or a super cheap way to do this is to get an easy and inexpensive sponge humidifier, and wet the sponge with ordinary tap water once a week in summer and winter, and once every two weeks in spring and fall.  The Planet Waves GH Acoustic Guitar Humidifier (below) is a good sponge humdifier, which is available for $5-10.