"Purge" is really only intended for use with fossil's "bundle" feature, bundles being essentially feature-rich patches which record all of the historical state of the bundle. (Yes, it's long been flagged as experimental, but it's also been unchanged since it was added so i'll look into getting that notice removed or amended.)

The idea of fossil's bundles is that a person not associated with a project can submit a patch in the form of a bundle, a developer imports that bundle into their repo and either retains it or "purges" it, leaving the developer's repo clone back in a clean pre-bundle state. That feature can, however, and sometimes is, used for "popping" the top-most commit from a repository (so long as it has not yet been pushed to a remote). So long as a local repo has not been synced with any remotes, and so long as there are no branch points to interfere with it, repeated applications the bundle/purge combo can be used to pop the top-most checkin multiple times, effectively wiping out as-yet-unsynced changes even if they span multiple checkins.