"That's as useless as tits on a bull!"
Plainly stated, tits on bulls are supposedly useless. Granted, I don't think that if I were a baby calf, I'd want to suck on my father's tits -- even if he did have them. The father can't produce breast milk anyhow, so why would he have tits?
Tits on a bull? Useless? No, not really!
A similar question can be posed: Why do men have nipples? Well, because they're sensative as shit, and it feels good when women (or men, whatever your sexual orientation allows), play with them.