Interesting to learn about that! I recently got into the development of GitHub actions and it seemed counter-intuitive to me. Primarily because the official typescript action template doesn't use it (https://github.com/actions/typescript-action), even though their simpler hello-world template does use it (https://github.com/actions/hello-world-javascript-action)

My other reasoning when I created this PR:

It makes the repository rather large for something that can be generated using commands
I read about the 'caching dependencies' documentation page on GitHub that talks about this, although to my surprise just now, it does not seem to cache node_modules after all¹² (https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows)
When I was looking up online on whether to commit node_modules the general consensus seemed to be to not commit it, however I can see that a GitHub actions repository might be an exception.

I see now it is not as clear-cut as I initially thought. I'll leave it up to you on whether to include this PR or not.

Footnotes

Remove and ignore the node_modules directory #42