The bonding/claiming aspect. I like that it's a more animalistic and base version of marriage and soulmates. One of my kinks is biting anyway, so I really love the alpha/omega fics where biting someone during sex bonds you to them and is a visible way to mark/claim them as your partner.

Also the "excuse" for someone being so desperate for sex they'll start rutting against anything trying to get off until they have physical contact with another person.

I don't really have any interest in the pregnancy aspect or self-lubricating assholes or whatever.