Typically these systems are set up to automatically snapshot on a periodic basis (say nightly) and automatically pruned after another period (say monthly) - if

Typically these systems are set up to automatically snapshot on a periodic basis (say nightly) and automatically pruned after another period (say monthly) - if there was massive activity due to ransomware causing a full disk in most such systems it'd have to be manually investigated, it wouldn't just decide to delete snapshots to free space. It certainly should help against ransomware in that an admin who was investigating an unexpectedly full disk, who's hopefully aware of such things, would probably notice that all the files on the drive were no longer readable and all had recent modification timestamps. And that's if a user doesn't happen to notice it first and report issues with their files to IT. As long as those snapshots last more than a few days as to provide sufficient time to catch and investigate the issue, ransomware damage is usually revertable much faster than backups with simple snapshots.