For masturbation, mostly.

Humanity is relatively unique compared to earth’s other animals in that the species’ females can conceive all year (minus the time when they have their period), and while it’s very hard to study this, it doesn’t seem like sex is particularly fun in most species, especially for females. This does help humans somewhat with forging long-lasting social bonds, which is definitely something that’s core to the human species, but it’s by no means the only way that a species can be instinctually inclined to form long-lasting social bonds. So to a hypothetical alien civilization, humans probably seem to spend a lot of energy on sex.