People find it a confusing idea to grasp that deleting things actually requires more space, either temporarily or permanently. Other comments here have gone into the details of why some modern filesystems with snapshotting and journalling and so forth actually end up needing to allocate from free space in order to delete stuff.
In a different field: In the early decade of Wikipedia it often had to be explained to people that (at least from roughly 2004 onwards) deleting pages with the intention of saving space on the Wikipedia servers actually did the opposite, since deletion added records to the underlying database.
Related situations:
* In Rahul Dhesi's ZOO archive file format, deleting an archive entry just sets a flag on the entry's header record. ZOO also did VMS-like file versioning, where adding a new version of a file to an archive did not overwrite the old one.
* Back in the days of MS/DR/PC-DOS and FAT, with (sometimes) add-on undeletion utilities installed, deleting a file would need more space to store a new entry into the database that held the restore information for the undeletion utility.
* Back in the days of MS/DR/PC-DOS and FAT, some of the old disc compression utilities compressed metadata as well, leading to (rare but possible) situations where metadata changes could affect compressibility and actually increase the (from the outside point of view) volume size.
"I delete XYZ in order to free space." is a pervasive concept, but it isn't strictly a correct one.
In a different field: In the early decade of Wikipedia it often had to be explained to people that (at least from roughly 2004 onwards) deleting pages with the intention of saving space on the Wikipedia servers actually did the opposite, since deletion added records to the underlying database.
Related situations:
* In Rahul Dhesi's ZOO archive file format, deleting an archive entry just sets a flag on the entry's header record. ZOO also did VMS-like file versioning, where adding a new version of a file to an archive did not overwrite the old one.
* Back in the days of MS/DR/PC-DOS and FAT, with (sometimes) add-on undeletion utilities installed, deleting a file would need more space to store a new entry into the database that held the restore information for the undeletion utility.
* Back in the days of MS/DR/PC-DOS and FAT, some of the old disc compression utilities compressed metadata as well, leading to (rare but possible) situations where metadata changes could affect compressibility and actually increase the (from the outside point of view) volume size.
"I delete XYZ in order to free space." is a pervasive concept, but it isn't strictly a correct one.