And, as Amazon says in the project description, "Sidewalk can also extend the working range for your Sidewalk-enabled devices, such as Ring smart lights, pet locators or smart locks, so they can stay connected and continue to work over longer distances."
Amazon doesn't charge anything for Sidewalk and the company says "multiple layers of encryption" provide privacy and security for your personal data. But Amazon has gotten some flak for automatically opting users in.
You can use the Alexa app to opt out, which is something Shelly Palmer, CEO at The Palmer Group, a tech strategy advisory group, said he plans to do in part because of the automatic opt-in and "I don’t like the idea of being a node on an Amazon peer-to-peer network and enriching the richest man in the world with my bandwidth, my data and my money."