It's a nice way to aggregate content creators who have good, but sparse or irregular posts - i.e. where it wouldn't be useful to pull every single one of them to see if there's anything new, you just have a single "inbox" for that, curated down so that it's not too much to read everything.

What alternative is there to rapidly scan through sites that have interesting content?

For the most part, I use RSS to read HN. Every now and then I will go to the home page to see if there is something I missed that didn't gain enough traction to make it to the feed.

Because it's a standard that allows an effective way to curate and browse lists of articles without having to fully commit to a single walled garden service. My primary method of browsing HN, for example, is via the RSS feed.