[Nagrom Nniuq]

Yeah, as has been said, that's just a normal thing you should always do. Some ROMs leave things behind, or rather, some ROMs don't overwrite everything. Doing it this way gives a clean start and that is how you should start all flashes, unless you want artifacts from your previous ROM showing up. Flashing 15 times isn't exactly a lot, so once you have done more and tried more ROMs, you will see why you should do this. The problem arises mainly from trying different ROMs, not multiple flashes of the same ROM.