Evolution is never a causal relationship even though school books often teach it that way. It's never that a feature evolves with an intent to make a change. It's always random mutation that then ends up being a positive thing and those animals then reproduce more.

So, in this case those people who developed the mutation ended up having more sex and thus reproducing more. The ones who didn't have it either had reproduced with those who did and thus their descendants ended up with it, or they didn't reproduce enough and died off.

That's how all evolution works in reality. The trait either makes it easier to survive, makes reproduction easier (or more survivable), or it makes the mate more appealing and thus more want to mate with the people with that trait than without.

Basically everything else is either connected to one of those traits, or there were other external factors that made that group more able to survive and reproduce and thus the trait had no real reason not to exist and just was random chance.

Organisms that enjoy sex reproduce more.

Organisms that enjoy having sex until genetic material is exchanged reproduce even more.

In mammals where reproduction can decrease fitness of the females (being pregnant means worse hunting, or worse at evading predators, and more needed calories) the females tend to be selective. There is an evolutionary pressure for them not to attempt to reproduce with every male they see. If they did, they would likely cut their own life short and the offspring may not make it.

So their orgasms become more elusive, and often include outside factors than the physical actions as well. For them the evolutionary pressure isn't just to have sex, it's to have sex with someone who could keep them alive during pregnancy, childbirth, and until the offspring are self sufficient. Not just capable of that, but willing. That's the whole "emotional connection" thing, and why most women aren't just hung up on looks.

But to clarify this is something way older than humans. Because orgasms have been around a lot longer than we have. Humans just have the language to d scribe the behavior it doesn't always mean we understand our own.

Same as every other evolved feature. Random mutation results in better reproduction. If orgasms felt bad, people would avoid them, and very few people would reproduce. For people who they feel good, they seek that reward, and reproduce more.

It doesn't really leave fossil evidence, and we're not anywhere close to genetically or taxonomically reverse-engineering the brain, which is where they happen. So no, we don't know "how" it evolved.

The fact we can masturbate also undercuts the "why" a bit, but it wouldn't be the first time a thing that evolved, and then was undercut by later developments.

https://onlinelibrary.wiley.com/doi/abs/10.1002/jez.b.22690

See above for a possible explanation for the evolution of female orgasms. Tl;dr: in humans, it may be a trait derived originally from induced ovulation in more evolutionarily distant ancestors and changed to become what it is in humans over evolutionary time.

In a general evolutionary biology sense, "why" questions are a lot harder to answer than "how" questions. Many answers to "why" that are often given as "that makes sense" are often untested "just so" explanations.

we would have to figure out how and why pain evolved. Our whole endocrine system is split between feel bad and feel good chemicals to get us to do things and not do things.