Well, I can't say for sure if if would benefit you, but the one thing I found useful about it is that it makes it easy to do code reviews without checking out the whole project.
At work we use GitHub, so to do a code review I have to check out the working branch or PR that I am reviewing, and review it in my local IDE/editor software. I have to do that because the web UI that shows the PR diff generally isn't sufficient to really review the code. I want to quickly jump to related files to follow the code path, keep the subset of files I'm referring to open in new tabs, get a project-level view of how the pieces fit together, and maybe step through some of the code in the debugger.
Now, the first two are possible just using a web UI but it's very unlike a normal code editing experience. Using the GitLab IDE interface makes it easier to do the first three of those things. It still cannot actually run/debug a program, but I presume that their goal is to someday enable that kind of thing, at least for some kinds of projects.
Today, all major git repo hosting services have rudimentary web-based single-file text editors — seems to me that adding IDE-like multi-file project-level editors is a pretty straightforward and logical extension of that.
So yeah, as a GitLab user, I want this. As a GitHub user, I want them to copy this idea and implement something similar. And if I were a BitBucket user...
