The original bell bottoms were worn by sailors. The reason for the design was to allow them to easily roll their pants' legs up when getting out of a boat near shore so as not to get them wet. It also made it easier to roll them up when scrubbing the deck. Or, at least, that's what I was told back when we were wearing these atrocities.
I saw a really exaggerated pair on a guy the other day and I just had to slap my forehead and go, "Oy vey! Was I the biggest Schmuck on Earth? Or what?"
Oh, and there's a very good song on the Derek and the Dominoes album, Layla and Other Assorted Love Songs, called Bell Bottom Blues.