Deleting a document, and why only admins can
Deleting a document is permanent and admin only, so a signed contract cannot be erased by accident or by someone on their way out of the team.
Removing a document from the vault is restricted to admins. Any member can upload a file and open one, but only an admin can delete it. That is deliberate: the vault often holds the only copy of a signed contract, and it should not be possible for a slip, or for someone leaving the company, to erase it.
Deletion is permanent. It removes both the file and its record, and there is no recycle bin and no undo. If there is any chance you will want the document again, download a copy first, then delete.
The usual reason to delete is to free up vault space, or to clear out a superseded version once you have uploaded the replacement. If you are an admin and simply want to tidy up, removing the old file and keeping the current one is exactly the right move.
If you are not an admin and need a document removed, ask one of your workspace admins to do it. The same people who manage the plan and the team are the ones who can delete from the vault.