When I could dance, it was fun. Especially in the disco era. My wife and I won a dance contest. Good exercise, too. We usually had some more exercise when we got home.
It's a celebration of life. People dance when they feel happy. Some just love to dance....I'm one of them. Even if I don't feel particularly happy, I will after I dance a while.
It's also a social barrier breaker. It may be just too hard to walk up to a stranger and start talking but it's easy to say, wanna dance? If they say no, it's not really a big deal.