So I finally managed to have enough time to sit down and type out this fairly lengthy response. For this post I'll be referring to master boxer Georges Carpentier. Carpentier had a fencing background when he first got into boxing, and there were several references along his career to his use of classical fencing methods and strategies throughout his career (for one such link, click here.) The "backbone" of Carpentier's boxing strategy as he refers to it in his book "The Art of Boxing" (circa 1921, same year he fought Jack Dempsey) was using his lead hand to feint or grab the opponent to set up his powerful right hand. Carpentier was not looking for knockouts with his left, just feints and set-ups for his right. He does share the similarities between a lead straight with fencing though and that it is a viable technique for some boxers. Carpentier was a very patient and adaptive fighter, and an excellent counterpuncher, hence why his style was different from the far more aggressive, crouch-fighting, in your face style of Dempsey's. Dempsey also didn't come from the privileged background Carpentier did and did not have that fencing influence; his skill was more developed in coal mining camps, bars, and backyard rings. So there already is some difference between lead hands for power and lead hands for set-ups at this stage in the game. Jim Corbett credits a powerful lead straight as to earning a knockdown against his win over John L. Sullivan (and breaking his nose) in 1892 so it's obvious they were hitting hard back then as well. I think moreso than anything it's just the fighter using it and how they see it as part of their overall game.