They don't grow back.
It is for showing, only in males though, because the breed originally comes like that before cockfighting was illegal. However with OEG's and OEGB's they weren't fought, I think it's just for frostbite and to show the birds' gamey appearance, otherwise it will just look like a small average rooster.
They don't need it though, yes, unless you plan to show them. (males)