Protects you against tracking through “free”, centralized, content delivery. It prevents a lot of requests from reaching networks like Google Hosted Libraries, and serves local files to keep sites from breaking. Complements regular content blockers.
Decentraleyes is no silver bullet, but it does prevent a lot of websites from making you send these kinds of requests. Ultimately, you can make Decentraleyes block requests for any missing CDN resources, too.