I make beer and cider. I prefer kegs, but I bottle a few as well. When I bottle:
-To bottle carb a sample while the rest is batch aging, for taste tests.
-Extra beer in fermenter...don't want to be wasteful.
-Small batch, not worth wasting CO2 on headspace.
-Going to a friends house.
-Like the batch and want to save a few for a later time.
-Insurance policy against having a keg empty unexpectedly.
-The urge to make a bottle label (sounds like a weird reason...but it's fun)
-Show my wife that I'm keeping all those bottles for a reason...after I promised they'd all go away after I started kegging
-Being surrounded by bottles makes me feel like a monk.
-Despite all the work, bottling is somehow cathartic. Even more so when you have this underlying knowledge that you don't have
to bottle beer.