dannye is correct in that the original bell bottoms were worn by sailors, but the reason is much more practical than mere convenience. Bell bottoms were/are a primitive life preserver. If you end up in the drink, you can slip them off quickly and easily, without the need to remove shoes. Then you throw them around your head and blow into the holes to fill them up with air. Of course, they don't work too well long term as you have to keep blowing into the holes as air leaks out.
Not perfect, but not bad for a little extra cloth on your pants.