You’re completely missing the point. Even Gitea (much simpler than GitHub, nevermind GitLab) is much more than a git backend. It’s viewable in a browser, renders markdown, has integrated CI functionality, and so on.
Even for my meager self-host use-case, being able to view markdown docs in the browser is useful from time to time, even on my phone.
As for the things I use (a self-hosted) GitLab instance at work for… that doesn’t even scratch the surface.