Disclaimer: I don't know shit about any of this.
Genes determine a lot of things. Traits, characteristics, these things can be more important than just good looks and good brain. If you're into the kind of thing, adaptation.
Haven't you ever wondered someone is good looking? Well, what do you think you think, well some of us at least, when you see one of them you think is well, attractive? "Damn, I'd do them, now."
Well, if you humans are supposed to reproduce, then why would we do it with the attractive people? What defines attractiveness anyway?
Well, supposedly general attractiveness is defined by how symmetrical one's face is. Basically, "Well, one side of their face looks like the other side, so, they're a fox." Of course, I'm sure people have their own preferences based on culture and other things, but it's generally a symmetrical face.
And, what determines a symmetrical face? Why, good genes! Holy sh*t, what an epiphany. Basically, we like to f*ck people with good genes. This, sadly, leaves out the rest of us with not so perfect genes. Well, wait a minute, that's not really fair? Haha, well, we're left out because we don't we have great genes. We don 't want our posterity to have bad genes right? I guess that's evolution for you?
So people with good genes get sex so whatever pops out of their vaginas also has good genes, and so on and so on, allowing the human race, or any other race for that manner, to keep good genes in, and bad genes out, letting us do whatever we do with good genes, adapt, evolve?
As I like to say, it's my sacred duty to omit my genes from the gene pool. (no offense to my parents)