Close enough JC - a bass player provides the bass line which is a fairly crucial, structural aspect of most types of modern music.

Think of it in term of the lower keys on a piano. The bass is essentially responsible for playing those notes and, at the same time, matching or accentuating the beat of the drum kit, particularly the large bass drum.

That said, it's extremely rare to have a band with a drummer and guitar player, yet no bass player.