Q: What are the benefits of practicing while standing up?

A: If you are going to perform standing up, you should definitely spend a considerable amount of your practice time standing up.  It is a completely different feeling from sitting with a guitar in your lap.  If you are a beginner or intermediate player and you've never stood up and strapped your guitar on while practicing, you're in for a bit of a surprise.  It is not the same.  Your sight line to your hands and the strings is different.  Your arm position is different.  Your hand position feels different.  You need to get used to it.  So if you ever plan on being a gigging guitar player, I highly recommend that you practice standing up on a regular basis.

This is even true for advanced players when preparing a new set for and upcoming performance.  After you learn the song, the goal of practice is getting muscle memory so you can perform perfectly even if your mind is distracted by new sights and sounds during the performance.  Since standing and sitting use different muscle patterns, if you sit all the time you'll be training your muscle memory to do the wrong things unless you practice in the same body position as you'll be performing in.