I recently had the need to subscribe to changes to a Github repo and it turns out it provides a feed for them. For rust master branch, for example, subscribe to https://github.com/rust-lang/rust/commits/master.atom.
I would say they are useful, specially against a relational database, when you have a ever-growing or dynamic amount of different entity types that are all related between each other. With a knowledge graph you only don't need to create new tables every time you define new entity types, you also can give semantic meaning to the relationships between them.
Offtopic: I'm seeing a trend of titles that, as a non native English speaker, always confuse me and take me a good couple of seconds to finally understand. This is one of them. It's like they use a comma where I would expect an "and" to be. I wish I had more examples to show, but I have seen a lot here in HN. Is this some new editorial style or was this always used?