Global gitignore is for idiosyncrasies of your computer, not for repo-specific stuff. For example if you use an editor that writes in-place backup or swap files (like .swp files with Vim's default settings), or you're on a Mac which writes .DS_Store files into every directory, you're going to want those ignored for every repo. That way you can comfortably work with other people's repos, even if they're not configured for your particular setup.

The global ignore combines with gitignore files in each repo. So if it's an ignore pattern that everyone working with that repo should have, put it in the repo.

I actually like it a lot for stuff that is user specific, not repo specific. I imagine someone could have a "build.log" to view the build output in some specific way that hooks up to a mini taskbar app or some other random shit.

I've personally added some files that don't relate to the project at all and has no reason for being inside a .gitignore and I've had to add it many times.

If you're contributing to a larger project, it might not be trivial to modify the .gitignore file, because you might have to go through a review process and so on. It might be easier to just ignore something locally.

It's not set by default i think?