It's a building the government didn't have any use for, so they let the National Building Museum use it to house (boring and expensive) exhibits. It's also used for events all the time. They were setting up for one today. You can just wander around the parts that don't have exhibits in them. You have to pay for those. And there's a really cool museum store there.
It's popular with parents of little kids. See the girls doing cartwheels here? It's a heated space where the kids can go to get out of the house.
