This is a profoundly useful plug in. It is so useful it justifies using Firefox without any other reason necessary.

For privacy - you can restrict social media companies to their own "boxes" - stop getting those ads that follow you around the internet. Also, keep your banking to its own area. More possibilities.

For families sharing a computer - each member can have a separate container to each access their own email accounts without log out/log in.

For power users - Keep your work and personal accounts separate, run side by side logins into the same service, etc.