Many male humans are sexually attracted to the breasts of female humans. Is there an evolutionary reason for this?
Short answer, we don't know.
There are several competing hypotheses but they are all nearly impossible to test. Behavior often suffers the same problem as testing, we can come up with a hypothetical reason but since we are the only organism that does it, we have no good testing options. We can't put people in controlled conditions for generations and see if their preferences change. Secondary sexual characteristics in general have problems with testing. Because there is no good way to differentiate reasons for a behavior. We still don't have a great consensus on why secondary sexual characteristics in humans exist, likely because many explanations can be true at the same time, humans have very complex behavior even in out instinctual behavior. It could just be a quirk of neurochemistry caused X to be attractive, followed by run away selection, and that by itself would create a selective advantage. It can be really hard to test why a preference would arise unless the advantage is obvious because the different hypotheses all behave the same way in practice.
Possible answers include:
-lifting the nipple away from the body to make suckling easier for flat-faced human babies with protruding noses.
-runaway sexual selection (which is also a component of many others as well).
-cryptic signalling of ovulation and/or pregnancy.
-all of the above
This is a hypothesis that I found in Desmond Morris book, The naked ape.
Basically, humans transit from a rear-penetration position to a frontal-penetration position. In this transition, the sexual clue performs by buttocks was perform now by breast.
In short, according to Morris: breasts are sexual clues because they look like buttocks.