This extension is excellent if you own multiple accounts for the same service and want to keep things isolated.

For instance, I have a personal GMail account and a work-only GMail account. It's nice to keep these things separate, because then you can use Single-Sign-On authentication without needing to continuously specify which email account I want to use for signing.

You can also associate certain websites with different containers, so that they are always opened in the appropriate container. Very useful as well, though it can lead to problems when you open the same website in two different containers (for example: I use GMail for both work and personal matters).