The manual explains it pretty well: https://www.scirra.com/manual/150/containers
"Containers are for making "composite objects", or objects made out of lots of smaller sprites (such as a turret and tank base making a single tank). Once in a container, picking the base also picks the turret, and vice versa."
For example I have a enemy spaceship that have a lifebar, shield bar, shield and a trail all in a container, I just need to place the spaceship on the layout and all the others show up. I have a everytick that keeps them where they go. If I change the healthbar in an event they update for all the ships with the right values of the ships since they are spawned together.
I never used them before, now after trying them out I use them on everything that have multiple parts.