Need to understand: weighted bobber?

[gforcetrivers]

Stopped in Wallmart to pick up some "slip bobbers" for a little crappie fishing, which I purchased. BUT, ALSO saw several "weighted bobbers". Dum me, purchased a couple of packs,....but why/how do you use a "weighted bobber"?. Old Fart , but looking to learn!

Bobber Down,
Gary

FYI

Weighted end goes on the bottom!