Because you choose the news sources and you see everything they publish on that particular feed. A good example here might be the blog of an open source project

Because you choose the news sources and you see everything they publish on that particular feed. A good example here might be the blog of an open source project that you rely on. They may make a post every month or two when new releases are made. You care enough that you want to be aware of the post, and you know it's unlikely that you'll see it on HN, but it's not urgent enough that you need an email. Another good use case is webcomics. You like to read every new xkcd when you have time, and you don't have to worry about loading the page and not seeing any updates. Friend has a blog or tumblr that you feel obligated to follow, but you don't want to ever have to load it up and see stale content? RSS is perfect for that. Like reading David Brooks when he has a new column, but wading through Paul Krugman and Maureen Dowd is a chore? Just subscribe to the RSS feed on his author page. It's also useful for other things, for instance jungle disk provides an RSS feed for backup jobs. So do many bug trackers.