I dont ever really visit them during gameplay, but I have built a couple for my sims to store their collections. I play legacy, but I change houses for each generation, so it's really hard to keep all the items that generations have collected over the years. So I build a museum and put different collections in different rooms so my collections and family pictures are all preserved (really good for looking back at photos of sims from previous generations).
They can be good if you put a bar in it, for a posh date/proposal venue, and if you put some activity tables in there too then it can be a place to go and work on practical skills. I have an archeology bench in my Salvadorada room, an easel in my paintings room, microscope in my science room, observatory on the roof next to my space room... Oh and some tents so my sims can camp 'Night at the Museum' stylie ;)
