When I worked in games we did exactly this but with perforce. All of our libraries were in a vendor tree and all in source. We slapped our own build over the to

When I worked in games we did exactly this but with perforce. All of our libraries were in a vendor tree and all in source. We slapped our own build over the top of them and checked in the build artifacts (perforce). If we needed an update, we updated their code and maybe our build script. It'd be nice to use submodules for this but I gave up years ago. The other big use is where you have your own libraries and you'd like to be able to share them across projects. My friend does game jams and has his own simple engine, he versions the engine and adds abilities and uses it across game projects.