I know the danger of thinking about magic products that solve a large number of separate/related problems, but... several large FOSS projects (Debian & GNOME at least) are in the process of moving some or all of their git repo, code review, task tracking, and/or CI/CD workflows to GitLab.
My why for this particular choice is pretty well stated in the GNOME blog post linked above: "The GNOME Foundation believes that the move to GitLab will encourage people with various skill sets, not just coding, to contribute to GNOME more regularly, as well as encourage contributors to explore other areas of the GNOME Project that they may not have initially considered."