If, as Lindy suggests, you are referring to the Suggestion report about local/external files, then the reason to delete them is that they are useless to anyone here, except to the owner of the original gedcom import (and even then, only as a reference).
Whatever genealogy program they were using, linked those files as part of their "sources", but the files themselves aren't transferred over. Only the references are transferred. Maybe the original owner intends to upload the images later and needs the references to keep track of what's been uploaded. But more often than not, they may no longer have access to those files either.