For example, I have Hacker News in my rss feeds. I certainly never read everything or even close to that. As a fellow HN user, surely you are not interested in every article here?
Because it is a very good aggregation of almost everything on the internet I care about. I can have feeds for tumblr accounts, youtube accounts, twitter accounts and whole subreddits and without actually having an account in any one of them or actually having to go to them everyday.
This ties in with my complaint with Reader removing social features. I don't want to go to G+ everyday to see my friends' shares when previously I could stay in the only ``social'' network I care about.
Because nearly every site has an RSS/Atom feed and therefore, I can visit dozens of sites at once without ever leaving Google Reader. Not just blog posts and news sites, but anything with an RSS/Atom feed like Facebook status updates, tweets, etc. It's efficient web surfing.