I ride dressage primarily, but when I do ride western, I might post for these reasons:
-I'm riding the horse in a english bridle, but a western saddle, to get comfortable with the horse in a more secure saddle, before using an english one. So these are hunt/dressage horses that I work at a fast english pace, so sitting isn't usually the best in the western saddles
-if the horse has a little spazz attack, I might post, because then I don't bounce around, and am able to just think about calming the horse down, and not about staying seated. This of course isn't the case if the horse is trying to buck, or if I'm trying to slow the horse down.
-if I'm just trying to cross the arena quickly, I'll do a fast posting trot.