They do it for most pets now too. They're useful for identifying runaway pets wherever they end up, and for stuff like https://www.sureflap.com/en-us . I have a few of the sureflap doors and they work well for keeping my cats on their own diets (controlling access to a combination of crates + auto-feeders).

It's the same (exact?) thing as far as I'm aware; rice-sized glass bit with a tiny RFID chip inside.