"So this is the electronic equivalent of a chain letter? If you want to utilize the processor in my computer, you can rent it for a fee."
No, it's charity. It's the contribution of unused computational power to help speed science and discovery in the search for cures and understanding of the inner workings of diseases such as Alzheimer's and Parkinson's.
Perhaps you can sell your spare processor cycles to the folders over at DU. They're lagging badly. (But they probably want to be paid, too...) ;-)