why would you be in a relationship where you dont love the other person?

Sometimes that’s just the way it evolves

maybe they need somebody to support them or they're settling for whatever they can get?

Other commitments unite you, children, Financial.
Many reasons

I wouldn’t

Great sex.

Some people do it because they don't want to be alone, others because of sex or other gain they'd lose otherwise.

Maybe if theres money involved or if I was being blackmailed.

Me personally? I wouldn't, I can't do it. I'm single lol