There are many benefits for both parties (the developer & the Hub team) to using a tool like Gitlab for managing the code of a super group.
Live site access can be very dangerous for even a very experienced developer. The use of Gitlab removes the need to access the live site all together. The developer can code and test in whatever environement they want, add their changes and the hub can pull in the approved changes right through the HUB admin interface.
Gitlab also allows the HUB team to monitor and approve code after the developer has pushed their updates. Changes MUST be approved by the HUB team before they can be moved to the live site.
Developers will actually work on whats called a "forked" copy of the super group repository. This means they they are working on their very own version of the super group code and can do whatever they want to it without affecting the live site super group or any other developers also working on the super group.
Managing the super groups by Gitlab (outside of the main CMS), allows for more regular updates. At any point after the hub is configured to work with Gitlab, any hub admin who has access to the groups adminstrator panel can update the the super groups code.
Along with a nice code browser/editor, Gitlab comes with an Issue tracker and Wiki section for each project. Each project has the ability to use those sections however these please.