Being in a Sauna will gradually raise your Sim's Fun and Social needs (if occupied with another Sim) to maybe: "Being in a Sauna will gradually raise your Sim's Fun and decrease any stress moodlets. It will also raise Social needs if occupied with another Sim."
It's a nice place for them TO woohoo (one 'o' on to).
That's all I can think of. I think the guide is a great read.
@Carl *Edited: A little off topic but sims can levitate well (not fall down etc) at Level 6 of wellness skill while being very focused when meditating. Not level 7 like everyone is reporting. And maybe even earlier but I haven't tested that. My sim also gained some guitar skill during this meditation... well a notice popped up saying so, fact is he already was at L10 guitar skill!
"While meditating sim's name had a flash of inspiration on how to improve his guitar technique. He gained some guitar skill"