tl;dr this allows one to link Pipewire filter chains (such as EQ) to specific devices automatically. Useful for system-wide EQ, noise filters etc, especially multiple filters together. In particular, it all happens transparently so you can just connect to your normal hardware output and it should just work.

You could already do this manually with some work, but this should be simpler and more robust. I suspect this will help with the Steam Deck too, which uses some filters to correct the built-in speakers, and this has caused bugs at times.