I could've rented this space out to another business. Instead I put in a gym.
Most tenants won't use it. But that's not the point. It's a magnet. They walk through, see how nice the space is, and want to lease something here. Same reason an apartment complex puts in a pool even though half the residents never swim in it. Nobody uses it, but everybody signs because of it.
Save this the next time you're deciding whether an amenity is worth the cost.