When uploading an image (or a file of another type) in an activity (e.g. mod_label), the file is accessible via https://.../pluginfile.php/\{contextid}/{pluginname}/...

To reuse this file (especially images) most trainers copy this URL and then use this URL in other places (e.g. the intro of another activity).

If you want to reuse the course and do a course backup, these URLs (with pluginfile.php) don't get encoded like other URLs (e.g. with view.php or sth. similar) and still point at the old target.

pluginfile URLs should also be encoded for backup when linking to the files of another activity. It should be possible to replace the values of the context id after the course restore is done (as plugins can do in restore_..._task::after_restore()