If this is all my fault, I'm sorry.
A gitignore file specifies intentionally untracked files that Git should ignore. Files already tracked by Git are not affected; see the NOTES below for details.
$ echo '*\n!.gitignore' > build/.gitignore
Personally if I need a build directory I just have it mkdir itself in my Makefile and rm -rf it in `make clean`. With the article's scheme this would cause `git status` noise that a `/build/` line in a root .gitignore wouldn't. I'm not really sure there's a good tradeoff there.
touch build/.gitkeep
git add build/.gitkeep
git commit build/.gitkeep
".gitkeep" is just a human thing; it would work the same if you called it ".blahblah".
So their pitch is that if you want to explicitly keep the existence of the directory as a committed part of the repo, you're better off using the actual .gitignore functionality to check in the .gitignore file but ignore anything else in the directory.
I don't find it amazingly compelling; .gitkeep isn't breaking anything.
Better yet just do the work. If you want make a commit in a branch that's destined to be squashed or something, sure, but keep it away from the shared history and certainly remove it when it's not needed anymore.
To manually install it, you must clone the repo. Then you have to download models into the right place. Where's the right place? Well, there's an empty directory called models. They go in there.
IMO that's an effective use of gitkeep.
The simplest answer is that sometimes other existing software that I need to use treats an empty directory (or, hopefully, a directory containing just an irrelevant file like .gitkeep) differently from an absent directory, and I want that software to behave in the first way instead of the second.
A more thorough answer would be: Filesystems can represent empty directories, so a technology that supports versioned filesystems should be able to as well. And if that technology can't quite support fully versioned filesystems -- perhaps because it was never designed with that goal in mind -- but can nevertheless support them well enough to cover a huge number of use cases that people actually have, then massaging it a bit to handle those rough edges still makes sense.