It is very simple. Storage / bandwidth is not expensive. Residential bandwidth is. If you can convince people to install a bandwidth-related software on their residential homes, you can then charge other people $5 to $10 per 1GiB bandwidth (useful for botnet mostly, get around DDOS protections and other reCAPTCHA tasks).
Thank you for your suggestion. Below is only our plans/intentions, we welcome feedback about it:
We are not going to do what you suggest. Instead, our approach is to use the RAM people aren't using at the moment for a fast edge cache close to their area.
We've tried this architecture and get very low latency and high bandwidth. People would not be contributing their resources to anything they don't know about.