What Is Single Sign-on at All?
Suppose you have a web application that people are using to do one thing X, but you are doing it great. For example, it would be a web store allowing to order a custom T-shirt printing by uploading some funny and pretty images found on the internet.
You are looking for ways to extend its functionality by adding some more capabilities for your users, but you don't want to lose the focus on this thing that you are doing best of all.