Files in the user draft id are only valid whilst a user is editing a form with a filemanager / editor present. A new draft itemid is generated at the point where file_prepare_draft_area is called to copy the files into the draft area when the form data is initialised. This draft item id is then passed back from the form (as a hidden element) and used to copy the files back out again by calling file_save_draft_area_files (there are also helper functions file_prepare_standard_XX and file_postupdate_standard_XX that wrap these and cover the most common cases).
So, to be clear, the draft item id is just a random number that is included within the form data at the point where it is used, and should never be needed outside of that form processing (draft areas are automatically deleted a few days after they were last used).