git-annex allows managing files with git, without checking the file contents into git. While that may seem paradoxical, it is useful when dealing with files larger than git can currently easily handle, whether due to limitations in memory, time, or disk space.

Even without file content tracking, being able to manage files with git, move files around and delete files with versioned directory trees, and use branches and distributed clones, are all very handy reasons to use git. And annexed files can co-exist in the same git repository with regularly versioned files, which is convenient for maintaining documents, Makefiles, etc that are associated with annexed files but that benefit from full revision control.