I think gitignore solves a problem that is hard to solve with the traditional tarball approach.

Downloading a tarball and running ./configure or make, editing a config file here or there, etc then running `make install` is the most common flow. Now days I find myself frequently editing the Dockerfile to make it to my liking. With a git repo, the owners of the repo have excluded all the local files, build caches, etc and you can keep pulling to get updates stashing and reapplying your local changes. With tarballs, you have to figure it out all over again. Lose your build cache (language dependent maybe), lose a change you made here or there, etc.

Grabbing git repos instead of just tarballs is useful.

A) You can update them, because you can git pull to fetch changes.

B) If you want to apply patches on top, its better to have version control so you can keep track of what you changed, especially useful if you want to rebase.