Let's say I read CSS Tricks. However, I don't give a crap about any articles written about Wordpress or PHP because I do not deal with Wordpress or PHP.

Using an RSS Reader I can filter out wordpress articles and only get alerted of new blogs that I'm interested in. This also means I don't need to check CSS Tricks daily to see if anything new has been posted, as my RSS Reader will alert me when anything is posted.

I use RSS as a "partial subscribe". Something I wish I could do on other services (e.g Youtube, subscribing only to a specific playlist)