Why would anyone want a threesome with their partners?

Honestly I just don't get it and I've seen girls on here talking about "my husband/Bf want a threesome, should I do it?" Or "I had a threesome and I got jealous blah blah" or the guy ended up cheating. Why would anyone want that? And why would you allow your partner to disrespect you like that? To me, it sounds like cheating without being guilty for it. Also if my partner wants someone else, I wouldn't even want to be with him. I don't mean it the bad way, I am just really curious how do people think about threesomes.