They're not. They are just a formalization of that pattern, with a very tiny extra feature where the model harness scans that folder on startup and loads some YAML metadata into the system prompt so it knows which ones to read later on.

The difference is that the code in the directory (and the markdown) are hardcoded and known to work beforehand.

Not really special, just officially supported and I'm guessing how best to use it baked in via RL. Claude already knows how skills work vs learning your own home-rolled solution.