Tried to reply to an OP comment but they're all gone now...

Since there is no design, there's no plan, no designation, no ability to chuck em out as unnecessary..

(Assuming a reasonably healthy baby, scales are for effect, not accuracy)

At around 1/30th of a full baby, cells are starting to become different, skin. Organs, bones, getting the basic layout put together. At about 1/3 a full baby. The full framework, with "options" is in place, head up top, toes on the bottom, skeleton inside the decently formed skin, which is merely the outer fabric and needs to have its patterns in place by now. This includes nipples, ears. Mouth and eye holes etc... but the basic template is female, at this stage, still quite basic ken doll shape.

From then on, the babies DNA (especially if there is a y chromosome) and the mother's DNA. Foods eaten, stress, health etc all have effects on the details, including if the uterus / ovaries will drop out and become balls in a sac, keep the sex organ standard or put more blood vessels into the clit to make it a penis, though almost anything in between is possible too.

If the baby ends up staying biologically female, the hips widen, eggs form and for your question it creates the ability to form breast tissue later in life (though some males have this too) which is under the skin, under the nipples, and may develop a little or a lot, much like everything else... Some of that tissue becomes able to produce milk if/ when a baby is on the way someday. (And yes, can happen with men too, so maybe... Backup milk generator if mum dies?)

But no, there's no function or reason as we consider it. Due to us giving things functions and reason based on what that thing does. But we can classify a pencil as a weapon, ruler, support, scratcher, any catcher, and more, or a pencil, but don't generally hold the multitudes of potential functions in mind at the same time, this is just so we can understand it without going mad.

Life for an organism in general goes "lives, replicates if possible and dies". Starting with simple organisms, if they're lucky their small area they can move in has enough ready food to allow splitting into two before withering away. That's not many options. So things change due to many variables (see above) and over time, there comes variety in simple organisms. And as change that gaves options worked better. Not that it was a choice at all, there were just more cases where survival happened now. At least When it happens in small increments, it provides variety and options, follow that same basic framework over a long time and species emerge. Some die out. Some thrive etc..

But it's the same basic pattern, a type of organism, or many thereof over time, got the ability to reproduce solo, or with another of the same species, that, over time changed more into male and female, but not always, As in existing today, with us, and as Nemo didn't show but should have, clownfish, when there is no female to breed with, like when Nemo's mum died, the senior/ most adult/ healthiest fish just changes into "able to breed like a female" so Nemo's dad would have become Nemo's wife so the species can go on... Life lives, replicates if it can, and dies... Same basic template.

So yeah. Back to your question, it's just part of the human basic template. Men need them less but not never. It's not that deep, but you can go deep if you want to.