Integration of community features improves visibility and situational awarness. Compare this to emails or IRC, or forums.
Of course, effectiveness of this is as good as the ability to manage the vast information that get generated in open projects like this. What's the point of having everyting in one place if it's hard for users to find the information they need, or if no one is able to properly care for the tons of issues raised from the community... Anyways, GitLab is a tool that helps one organize and tie together these streams of project information.
What do you see being some benefits over the older tech? Because when I think "community features" I'm picturing something like Tweetdeck for devs (which, in fairness, may be completely different from what you're picturing). Basically, this repo I'm watching had these updates. That sounds like the same thing I get in email from an issue tracker, complete with comments from others watching.
Think of a remote-first collaboration arrangement. In such scenario the concentrated communications become vital for collaboration efficiency.
'Old' approaches could work too, emails can be typed and sent with a right set of CC, also IRC can be set up for chats, teams can roll their own messaging tool of choice.
It's all about policies and consistency in adhering to them. When these features are devised together as part of a tool, it offers to a client those policies out of box. Thus, feature planning or collaborating on a merge request becomes more transparent, perhaps more real-time, when the tool supports the 'social' features.
Just a feature as simple as "@user" mention notifications may promote the level of collaboration.