What's the point of b guitars in bands?

The b holds the foundation for what the guitar (or keyboards, or vocals) is doing. At it's most basic level, think of any AC/DC song- the b rides one or two notes while the guitar plays the main riff. In a lot of metal like thrash, the b doubles what the guitar is doing to add the 'strength' behind the music. In other styles, the b might play a line or a melodic part behind what the rest of the band and vocals are doing- Bob Marley's bist, Familyman Barret, is a master at setting the groove and the feel of the song- he's kind of the most important part of Bob's band (aside from Marley himself, of course). Next time you hear something by Zeppelin or Floyd, check out what the bass is doing behind the guitar and vocals- it adds a whole different dimension to the music. Better yet, watch some Zeppelin videos on Youtube- you'll see just how incredibly full the band sounded with just one guitar and a bass part. Of course, it helped having John Bonham playing drums... or watch some Rush stuff- amazing.