It’s just a web based client instead of a desktop one. And it can usually output its own RSS feed that contains your other feeds so you can hook any RSS desktop client on any device to it.

It makes more sense to have a server downloading and consolidating the data from the various sources, rather than syncing and downloading from dozens or hundreds of sources to build the feed in real time.

It's technically possible to do it all client side, but it would put more load on the RSS sources, and be a much slower user experience.

The idea is to imitate the experience of something like Feedly, an RSS feed you can access from anywhere on any device, recommendations, all that... Which is overkill if all you want is just a simple program that queries for new posts every x hours.

Cross device synchronization, battery optimization for mobile devices, faster update speeds, etc.