Lets you run your actions locally. I've had significant success with it for fast local feedback.
I tried this five years ago back when I was an engineer on the PyTorch project, and it didn't work well enough to be worth it. Has it improved since then?
I tried this recently and it seems like you have to make a lot of decisions to support Act. It in no way "just works", but instead requires writing actions knowing that they'll run on Act.