Here are reasons I've found it ideal to use joints.
-you're out and about and don't want to carry a pipe around.
-you reside somewhere you don't want someone else finding your pipe/bong/paraphernalia.
-you're smoking socially, have a limited amount, and don't want to keep packing a bowl.
-you live somewhere where cops give you shit about paraphernalia, so you want a way to smoke without post-session evidence.
-you fucking felt like it.
Thanks for asking, OP. It's always nice to see new perspectives!