Well, you now know how to restore an ORIGINAL_FMT.
As for why it is created -- calibre does not track multiple copies of the same format. However, the ORIGINAL_FMT facility is there for the one and only, sole purpose of helping you revert from a conversion gone wrong.
You can, of course, abuse that for other reasons.
But I will note that conversions always work from the ORIGINAL_FMT if one exists, and create a new FMT.
You are meant to delete the ORIGINAL_FMT once you verify the conversion worked.
You want to track revisions, invest in a Version Control System.
Yes, we have had people requesting version control ("track changes") in calibre.