They say that Jacobus de Cessolis from Italy came up w/ the concept & foundation for the 2-step doh-see-doh for today's modern day King move when castling. The King could go from e1 to g1/h1 where the Rook went from h1 to f1/e1. IOW's, there was a choice for both pieces. This "free castling" they called it, held up in Italy until the mid-1800's but France had made their own rules by then (which reflect today's castling movement) in the mid-1700's or about 100 years earlier. I guess news travelled slow back then along w/ how the Italians have a history of desperately clinging to tradition like jengy and his BFF Dark Army.
It was all about trying to enrich the game. But looking back ?....I think western Europe actually dulled it by not allowing uncastling. So. Maybe now 100+ years later ?....we change that rule. Or at least leave it to the players to decide if their game should be governed by it. Kinda like how McDonald's introduced the Apple Pie way back when. They let it be tried at lower levels and watched it gain popular traction.
They say when when the piece movement rules got consolidated a long time ago, they allowed the queen, rook, & bishop to go full range instead of two or three hops. This made the fighting ability of the King mirror a fat old sweaty stinkpot w/ a turkey leg in one hand and a mistress's thigh in the other.
So it had to be tucked away w/ a double move (as it trembled like a hairless chihuahua) behind some sacrificial foot soldiers & a fresh war-promoted Rook. Also, it complemented the other now-happy long-range pieces as they could quickly enter the foray w/out that useless fata$$ in the way.
And yes. The 1862 BCA rule impact was extremely significant.