Normally when stashing changes, using git stash, git is only going to
stash changes to tracked files. If there are any new files in your project
that aren't being tracked by git, they will just be left lying around.
You might not want these untracked files left behind, polluting your working copy. Perhaps these files change your projects functionality or affect the outcome of tests. You just want them out of the way, for the moment at least.
And so along comes the -u or --untracked flag.
$ touch new_file.rb
$ git stash
No local changes to stash
$ git stash -u
Saved working directory and index state WIP ...