But even if it is, it shouldn't prevent installing released debs you find for example on GitHub repositories.

Some things we would want to install aren't in the official repos. Downloading the deb file is a solution to that for newer users.

A lot of software wont be distributed with a PPA to add.

Additionally, debs are useful for offline installations, with apt you're able to recursively download a package and all of it's dependencies as deb files, then transfer those over to the offline machine and install in bulk.

That being said I've never had great luck with the software center, it's always felt broken. I'll typically just dpkg -I <pkg>.

Yes, depending on the context

You should try MX, it's Debian based, and they have their own repo full of .deb, up to date, never break