Buggish: Sharing folder breaks IIS

I do not understand why sharing an IIS directory breaks IIS. The worst part is that I cannot undo this. I cannot unshare the directory and have IIS work again. And when I compare to other IIS directories, the permissions look exactly the same.

There are several resources on how to resolve this which typically boils down to adding IUSR or IIS_USERS to the directory. But what I am interested in is how do I return back to before? How did IIS work before sharing? I can understand that sharing the folder changes the permissions, but what did it change?

I check to make sure all the users are the same under the security tab under the folder properties. Is there another way to apply permissions to a folder? There must be, right? Otherwise, I should see a difference.

I also checked the web.config file which is the main reason that IIS breaks because it no longer has access to read that file. It makes little sense that this would be it because I can always create a new web.config file which happens often with developers because they want to backup the original then drop in the new file.

I also checked IIS Manager, but nothing seems to make sense there. Authentication is using Anonymous Authentication and Application Pools uses IUSR. Oddly, adding IUSR does not fix my problem. I had to add IIS_USERS to fix the problem. Why? I have no clue.