Why might you want to do this? Two reasons:

1. Fast Feedback - Rather than having to commit/push every time you want test out the changes you are making to your main.workflow file (or for any changes to embedded GitHub actions), you can use act to run the actions locally.
2. Local Task Runner - I love make. However, I also hate repeating myself. With act, you can use the GitHub Actions defined in your main.workflow file to replace your Makefile!