Growing up a lot of us were sent to the basement rec room to play or hang out in. This was the room that was built for the kids, all games were fair play and you didn't usually have to worry about breaking any of mom's nice and expensive things.
Nowadays, things haven't really changed all that much. The basement is still the room kids gather and play in, but they look a lot different. In many new high-end homes, the downstairs has been upgraded to swanky sports courts.
In areas like the mid-west, it's a no brainer to add these to the home due to the amount of time we have to spend indoors during the winter. In addition to sports, an indoor court is versatile and can be used for a lot of other things, which makes it a better investment than a pool or home theater. Kids can blow off steam with a game of basketball, Rollerblading, or fort and tunnel building with exercise mats. They're contained, they're safe and they can be down there for hours. Which makes for pretty easy parenting. Courts are also a great spot for family gatherings and projection screen viewing if they are set up properly. And now, a number of builders are adept at both new construction and room additions.