Nowadays the stuff on GitHub is simply a mirror. Originally, they used GitHub primarily. Why they used GitHub was explained by Sytse (or Sid) in 2013:

"We're just going to stay where the people are. [...] GitHub is where the contributors are."

You can hear him yourself in The Changelog #103 podcast at https://changelog.com/podcast/103. Again, this was from late 2013.

Backup.

That's fine, but why would GitLab execs not insist on keeping it private?

Doesn't inspire confidence in the platform...

I started to refute this, but as I reviewed my post, I came to the realization that I have less confidence in the platform today than I did 12 months ago. Don't get me wrong, I like the Gitlab team, if you could call it that, for their transparency under duress, and how they kept us abreast of the troubles. However, the 'people' is not to be confused with the 'product', and if you're one of the users affected by recent events, then you may be justified in not giving a positive review of their service.

It's just a mirror nowadays. In the past we used to accept merge requests via this mirror as well.

Automated builds with Docker Hub as one of the reasons, IIRC.