We got a wet bag once we wanted to be out more often.
It wasn't at all needed when we were mostly home, but our little baby dude soaks a lot of diapers, especially with pee, and we didn't want to use disposables when we're out unless necessary (if we ran out of cloth, for example). Every couple days I wash this in the diaper laundry as it starts to smell pretty funky if we actually use it. And we use it every time we go out.
I couldn't imagine needing one that was as big as the one I purchased, but at 12.5 x 15.5 inches, it's just right. This would be an item that would be worthwhile to have two of, one for the wash and one for using.
Here's an image of a wetbag like ours:
Features:Strap with snap for easy carry along & attaching to other bags. (so many bags!)
Zippered front pocket for "clean" things, such as our wipes, spray bottle, shells, and inserts. I always throw in an extra onesie & baby pants.
PUL liner to keep wet stuff inside.
This one is from Planet Wise. I wanted to buy one locally made but they all had princesses and I'm so not a fan. :p