I don’t think it’s a matter of relative numbers enabling anything new; just less cognitive overhead when it comes to doing mental arithmetic for figuring out how many lines to delete or change or whatever, especially in large files when line numbers get to triple digits. In this case, it’s not what it enables; it what it takes away.
It would be different if relative line numbers required a plugin, but as a built-in feature, I don’t see it as not vanilla.
We often hear about stories of the supposed learning curve of Vim (see The Vim Learning Curve is a Myth, which I mostly agree with); part of that is because people want instant results and are impatient and don’t feel instantly more productive compared to their previous editor. Being able to enable a few configuration options on top of a vanilla setup would help keep Vim newbies engaged until that lightbulb that longtime Vim users have experienced turns on.