When you update an issue and have "Attachments" available in your current screen, the user may attach some file and then hit the submit button.
If the workflow step fails, for example because a validator checks some conditions and fails, the user is returned to this very screen with all fields intact but the attachment list, so the user has to attach all files again

See the attachments:
Picture 1: I attached 2 files and submit the form
Picture 2: The submit failed, the list of attachments is empty.

Also, is there a way to show the existing attachments in such a form, so the user does not an already existing one?