- You boot your PC and use it.
- Save files to a file store (local or network, whatever)
- PC configuration doesn't change.
- PC resets to original state on next boot.
- PC gets an atomic update to new system state.
It makes desktops and servers way more reliable as their "state" is configured outside of manual changes to local files, which in turn makes deployment and reinstallation robust and repeatable.
Companies tend to use configuration management systems (e.g. Puppet) for this, but the systems deployed are not stateless so the config management systems have to keep checking all files for state (and resetting changed files etc.). A stateless system wouldn't have that problem - you set something, it is deployed. You deploy a system update, you know that all systems will be in a known state, update will succeed.
You can spin up any number of identical systems quickly and easily and know they will all work because the underlying OS remains "locked" - and so local changes can't be made by users or malicious programs which might break configuration or introduce further security risks.
It's a game-changer for large deployments, both servers and desktops. Clear Linux was originally targeted at server deployments (e.g. VMs, clusters, containers, Kubernetes, etc.) but if it's possible to do this with desktops too... boy howdy.