MAME supports more systems.
FBNeo supports homebrews/hacks MAME doesn't support.
MAME has more of a preservation focus, while FBNeo has more of a gaming focus.
It's not necessarily redundant to use both, it can be complementary. A lot of users like FBNeo for the things it offers (good savestate support, netplay, runahead, rewind, ...) and use MAME only for the games FBNeo doesn't support.