Nope.
They're most likely purely decorative. Selected by sexual preference. Cave men were boob men. Other apes don't have knockers and they need to produce the same relative amount of milk.

Mmmmm norks.
( Dr. Shambolic je suis charlie, Sat 23 May 2009, 19:00, archived)

They don't?
filmdrunk.com/ul/347-MonkeytitsW.jpg
( Lord Gnome, Sat 23 May 2009, 19:03, archived)

Ah yes, but the norks of all mamillia swell when rearing their young.
I would have thought that larger norks would give a hint to the amount of swelling that would occur. I'm just speculating, but if I can use science to justify staring at norks, then so be it!
( SuperMatt, Sat 23 May 2009, 19:03, archived)

Or maybe we see large norks as being "already swollen" so it becomes attractive because it's proof of fertility

( SuperMatt, Sat 23 May 2009, 19:05, archived)

I thought you preferred budding norks.

( Lord Gnome, Sat 23 May 2009, 19:05, archived)

They swell because they've got milk in.
Little boobs swell too.
( LimeyTreat Bitch. Lover. Child. Mother. Sinner. Saint., Sat 23 May 2009, 19:11, archived)

Like I mentioned above
it may be the idea of already being swollen that attracts males. I could be wrong, but hey, it's a theory.
( SuperMatt, Sat 23 May 2009, 19:13, archived)

So that's why men like big swollen bellies on women
it's clear.
( Flapjack I spay a little mare for you, Sat 23 May 2009, 19:16, archived)

That's not what I said!

( SuperMatt, Sat 23 May 2009, 19:17, archived)

Some men do
It's a fairly well-known fetish to prefer pregnant women, just as it is to prefer lactating women.
( Lord Gnome, Sat 23 May 2009, 19:22, archived)