If you happen to need more space, or after you back up your Mac to a permanent storage, the temporary backups are automatically removed by macOS and the disk space is reclaimed. Note #1 This hidden space is taken by Time Machine’s local snapshots, which typically pile up until they take ~80% of available disk space. If you are reading this, you must be wondering why there is still many GBs of hidden space on your disk, even after you scan as administrator? The short answer and the good news is that this is by macOS design.
Here's the information from DaisyDisk's site:
