Well, Cosmopolitan asked a bunch of doctors, and the answer is enlightening!
Epic side note that we've learnt: guys can experience nipple orgasms too, but we'll get to that.
Dr. Tara Bagen explains that nipple development happens approximately a week before sexual differentiation occurs in embryos. This is why both male an females have them!
Dr. Joseph Cruise also explains that by the time the Y chromosome and the hormone testosterone (which is responsible for the peen and testicles) happens in male embryos, the nipples have already been formed.
So they really are useless.
BUT - they are just as sensitive as female nipples, which is why (and how) guys can experience orgasms from them.
Dr. Joshua D. Zuckerman also told the publication that both male and females have the same nerve bundles in their nipple.
As Clinical Sexologist Sunny Rodgers explains, "the harder the nipples become, the more sensitive they are to direct stimulation.
"Much like the moment before a woman orgasms, don’t stop but don’t keep going full steam," she explains.
It's funny how the reason behind girls nipples having to be hidden away is because they're defined as 'sexual', when guys can use them for their sexual pleasure too. However guys don't lactate from their nipples and create life food for babies. (However if they do, they're advised to see a doctor asap).
Interesting, interesting, interesting.
Who would have ever thunk it?