Well, looking at proper swapping as opposed to group scenes and orgies, it’s really about the shared experience. Monogamy is great, but there are some things that you can’t do, and can only share vicariously. Being able to more viscerally see and hear a partner being fucked, or otherwise engaging in sex, you get a fuller perspective of how they respond, what they’re feeling.

Sure, when you’re with your partner, you have some ability to see what they’re experiencing, but it’s different.

Plus there’s the aspect of being able to interact with the member of the other couple you’re with. Being able to share the experience of watching your partners do their thing enriches your own sex. Just the experience of looking over at your partners, then looking back and sharing a smile or a laugh, or the enhancement of pleasure is such a wonderful moment of human connection.

Even taking turns, where one pairing has sex while the other watches, and then the second pairing uses the arousal from that to lead into sex, which often feeds back to the initial pairing so they go again is a very powerful thing. If you pick the right couple, it deepens not only your relationship, but the friendship between couples. Which may not seem hot on the surface, but the best sex happens as partners learn each other. So an extended friendship can make the sex amazing between everyone involved.

The camaraderie of it is nice as well, though that’s not the hot part. Being able to congratulate friends on how well they’ve engaged with your partner is lovely.

And, like you said, watching a partner fuck is better than porn with them. It’s immediate and real in a way that recordings can’t match.