$1 = more ears on your song!
Then if someone likes your stuff, they'll be more likely to buy your *actual* music or other merch, or go see your show if they're local. And that's profit that's more than the $.30 per dollar RBN will pay you.
But it's not about the money. It's about capturing the hearts and minds of the listening public. Many of us find that infinitely more valuable.
(And yes, before people ask, we *would* have given our stuff away for free if it had been an option!)