If you value your privacy and want to mess with how we are tracked online, this add on is great.
If you have several users (who trust each other, I suppose) sharing the same browser instance, then this can save you a lot of logging in and out (yes, I know, not a secure practice, hence trust is implicit, but secure or not, it happens).
Both of the above I do routinely, and I have really appreciated this add on for that.
For me, though, where it really shines, is in the following too situation:
- I play a web game where it allows me to be logged in to several instance,
- opening a new container is often a fast hack that allows you to bypass the article count in some websites.

So, in sum, if there is any situation where you would like one browser instance to behave as several instances with separate contexts, this is for you.

And huge thank you for the developers, for coding this, and for doing so in a Free/Open Source app! May your kindness and dedication come back to you and bring you joy!