I used it to build a (now permanently unfinished) lightweight DAG runner that uses only the Python standard library, with the intention that you can just copy the .py file into a project and use it without installing anything other than Python on your system. I think it might be of niche use to some people, but I personally wasn't even dogfooding it, I was just scratching an itch.

The purpose of adding it to the standard library was to implement linear MRO for classes: https://bugs.python.org/issue17005

It’s a common algorithmic need.

Not as common as array sort. But still common.