Larger projects need a way to collect bug reports, feature requests, maintain documentation, and set up milestones and release schedules, at a minimum. Ideally you want all of this integrated into your vcs, so that when a developer decides to push a commit related to a specific bug or feature request, that commit gets attached to that bug or feature request.
When these are set up and run well, they are very powerful for mid-to-larger organizations.
Finally, there's the fact that many more users will be familiar with GitLab (or GitHub, BitBucket, and so on) than they would with each and every opensource project's flavor of bug/issue/feature/commit tracker. For instance, I know exactly how to find the "releases" section on github quickly - if KDE wrote their own bug/issue/feature/commit tracker, I'd have to find the "releases" area and remember where it is every time since I don't use that UI as often as I use github.
Well, the question is pretty much answered in the actual article. From the looks of it, this seems to be responding to just the title without having actually read the page.
Having to find a bug tracker, create an account, log a bug is a lot of hassle for a random library or small project. I know I've given up raising bugs for some things as I couldn't be bothered with the rigmarole of sign-up, verification, etc... On Gitlab/GitHub there is much less friction because I use both regularly and and can use them for multiple projects, large and small.
It uses maildir files in a hidden directory, which can be tracked like any other files; e.g. when committing a bug fix we can also commit the closing of the associated issue. I've also configured Emacs to make using it even smoother (opening issues/comments in a derivative of message-mode for syntax highlighting, with a C-c C-c binding to save and stage; I've partially finished a derivative of tabulated-list-mode for browsing issues interactively).
I remember that UI change. At the time HN was really starting to look like it was getting gamed, with comment scores having too much influence on popularity. HN's response to hide scores seems to have helped.