It didn't used to not be necessary and then it became a thing. My guess is that it is mostly marketing but I could be wrong. Personally I don't use it on anything I build but I do know guys who swear by it. It would cost a ton of money to answer the question and the answer would most likely piss off some big boys who make a lot of money selling things so I doubt the test will ever be run.

Best bet if you want an honest answer is to find an engineer who works in engine development at an OEM and ask if they use break in oil for production engines. From what I've heard, they don't. Factory fill with synthetic and ship them out the door.