Bilbo's motivation being helping the dwarves find their home is indeed a nice counterpoint to Frodo's "save the world" task [Sam's being "save Frodo" lol]... it's a very "hobbity" reason, one that even the less Tookish among them might understand. I see echoes of WWI and WWII in there as well... not everyone was there to stop global evil, but many just to help out folks who'd lost their homes and needed someone to pick them up.