Couchbase (not CouchDB) gives you a whole bunch of things. You get the embedded document database, sync/replication, conflict management, auth, scale, data routing, querying, and more. Not every project needs all these things, of course. It's just damn hard to get this stuff right when you do need it.

I also personally find it way easier to use than the standard stores on Android and iOS for on-device persistence. 2.0 is coming up and should simplify the use a bit, too. So as it stands now I'd say the main sweet spots are, ironically, around the two ends of the spectrum. Easy to use for persistent local storage with no sync, and the industrial strength solution when used with sync.

(I work for Couchbase on the mobile side.)