I like Gale McMillan's story of how "break in" came about: A custom barrel maker recommended all this shoot one and clean, shoot one and clean for some rather large number of rounds. I disremember; 40? Anyhow, when Gale asked him about it, the guy said that it was forty less rounds of barrel life before the customer came back for another barrel.
So, just guessing, the recommendation got spread through the shooting fraternity, and some guys at the gun companies heard about it and believed in the necessity. Well, nobody was ever born an expert anything, no matter where they work. Once some such thing is put into writing, and from a gun company, it becomes Biblical. Gun writers will parrot the instructions, regardless, so they can keep getting guns to test or be invited on hunts. "If it's in print, it must be true."
Me, I never heard of it until here at TFL, back around 1999. My uncle never mentioned it, and he got into the '06 game in the 1920s. I got into an '06 in 1950. I did gunshow tables from around 1970 until the mid-1990s, and never heard anybody ever mention "break in", anytime during the numerous bull sessions about shooting. Or at Lord knows how many gunshops...
But I figure that folks oughta do what they think is righteous.