Stash is just a convenience method. Since branches are so cheap and easy to manage in git, I personally almost always prefer creating a new temporary branch than stashing, but it's a matter of taste mostly.
The one place I do like stashing is if I discover I forgot something in my last commit and have already started working on the next one in the same branch:
# Assume the latest commit was already done
# start working on the next patch, and discovered I was missing something
# stash away the current mess I made
git stash save
# some changes in the working dir
# and now add them to the last commit:
git add -u
git commit --amend
# back to work!
git stash pop