But why does Mozilla think they are so important that extensions shouldn't use them?

I should have said something like, help me understand why removing all these entries is in my best interests, so I can access the websites run by Mozilla as a user intends.

Some Mozilla websites like this forum and also the Mozilla Account website have special permissions that allow accessing and modifying data on your computer. If you remove this restriction then all extensions are able to access/abuse this data as this is an important security feature.

extensions.webextensions.restrictedDomains protects Mozilla domains with special powers from being abused by extensions. See Restricted domains:

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts