If you're playing a gig on a big stage, outdoor festival, arena, or large venue, you're probably going to be faced with having to run your guitar amp into the PA system.
The purpose of this, of course, is so that your guitar will be heard in a situation where the amp alone is not enough.
There are also times, even in a smaller, low volume venue, that running the guitar amp through the PA is a good idea, just to add some "presence", and to fill out the overall mix of the band.