It's not doing nothing though, you're actually training your mind, but because you have to use your mind to train your mind you have to do so indirectly. It's very much like Mr. Miyagi training Daniel karate by getting him to paint fences and wax cars. The skills develop sub-consciously, but with practice, the skills still come.
Learning how to "empty your mind" or more accurately to simply be a thought-spectator while avoiding getting swept away by thoughts as they flow through your mind may seem like "doing nothing". But imagine how useful it is to have those skills when you're trying to concentrate on something else? If you can avoid being swept away by random thoughts when you have nothing else to hold your attention, think you'll be better at concentration when you do have something? And that's just the tiniest benefit.
Learning how to "empty your mind" or more accurately to simply be a thought-spectator while avoiding getting swept away by thoughts as they flow through your mind may seem like "doing nothing". But imagine how useful it is to have those skills when you're trying to concentrate on something else? If you can avoid being swept away by random thoughts when you have nothing else to hold your attention, think you'll be better at concentration when you do have something? And that's just the tiniest benefit.