Because you're not uploading every song to YouTube.
Let's say that I'm searching for <Song A> - and I listen to it on your channel. Now I'm in the mood for <Song B>, so what do I do?
Oh wait, in your description you said that I can download music from your site... I think I'll go there!
It's sort of like being a drug dealer. You give them a full sample of what you have to offer. (In this case, one song.)
Then they're hooked and they want more... so you help them by bringing them to your site.