You might have interests in many different domains or subjects and it's nice to have a live feed of each one separately vs all clumped together as one. Tags sort of help, but IMO columns make it easy to sort by tag, while still letting you see multiple at once.

RSS is also a great candidate for event feeds similar to tweet deck - where you're less interested in reading in the reader, and more interesting in organizing different event feeds that you can curate to ideally alert you to important things quicker - security bulletins and space launches might be good examples.