It means that if someone breaks out of your container, they can only do things that user can do.

Can that user access your private documents (are these documents in a container that also runs under that user)?

Can that user sudo?

Can that user access SSH keys and jump to other computers?

Generally speaking, the answer to all of these should be “no”, meaning that each group of containers (or risk levels etc) get their own account.

Landing gears are usually designed to drop by gravity (or manual hand cranking) alone if there’s a hydraulic failure.