The BATF ID number is more for situations where someone with the same name (or who stole someone's ID) has a bad past, and the innocent party doesn't. Kinda like two Bill Smiths. One is a LEO, and one is a convicted felon. The LEO goes to buy a handgun, but has been confused for the felon in the past. As such, the LEO contacts BATF and gets the ID number. This then speeds his paperwork. Something of an extreme example, but you get the idea.

When I was after my last civilian security security clearance in the past, my old military clearance (with all the "bell, whistles and abbreviations") actually made the puny civilian "secret" harder to get. I haver NEVER figured that one out.