Origin was somewhere around this 1989 email:

"You never sent me a response on the question of what things an app [application] would do that would make it run with MSDOS and not run with DR-DOS," ... "Is there [sic] feature they have that might get in our way?"

and 1992 one:

"What the [user] is supposed to do is feel uncomfortable, and when he has bugs, suspect that the problem is DR-DOS and then go out to buy MS-DOS,"

Microsoft did, petition to them to change their policy. Or... use a system that does not treat you like the product.

Both versions of Edge are used for webviews in native apps, removing it would break lots of them. Even not UWP ones, since both the Spartan webview and the new Chromium-based webview2 can be used in normal desktop apps too.

For example, the auth0 SDK for WPF uses the Spartan one for login screens

Yes it is very common: Ie on windows previously, Safari on ios, Chrome on Android. Or am I understanding it wrong? It's often integrated in UI frameworks.