RSS Feeds are practically useless. As far as I know, they're only really used for bots to catalog stuff and the like.
They keep you up to date whenever something new is posted to a website, or a specific part of a website (depending on how the website has the RSS feeds set up), like i'll see when Ross Scott posts a new blog post on Accursed Farms, or when RandoWis puts up a new comic or blog post
RSS feeds can be pretty useful for notifying you of something you want to keep track of (provided the website has a feed available), and RSS Feed Reader even lets you export to a file what feeds you're subscribed to in case you ever reinstall the extension or your browser, then you can import the file and it'll be like nothing ever happened
It also has a light and dark theme, which is always a nice thing to have