It allows you to use `git pull/push' without specifying the remote. see: http://mislav.uniqpath.com/2010/07/git-tips/ Section "Push a branch and automatically set tracking"

It creates a tracking branch, and so git knows how far a branch is relative to a remote branch.

       -u, --set-upstream
           For every branch that is up to date or successfully pushed, add upstr
           eam (tracking) reference, used by argument-less git-pull(1) and other
           commands. For more information, see branch.<name>.merge in git-config
           (1).

--man git-push