Google updates their fonts. They fix hinting, adjust metrics, and fix bugs. When you download a font today, you freeze that version. In three years, your website might look distinctly worse than the live Google version, or worse, render incorrectly if a browser update changes rendering logic.
When Google hosts the font, a user visiting your site likely already has the Roboto.woff2 cached from the last site they visited (because everyone uses Google's CDN). When you self-host, you lose that shared cache. The user downloads the font fresh from your server. This is a performance regression . google web fonts free download
Google Fonts is a service, not just a library. When you download the font, you are opting out of the service. Make sure you are ready to run that service yourself. Google updates their fonts