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.
Another interesting thing to note - I finally dug up my old copy of "The Science of Boxing" by Professor Mike Donovan (circa 1893), an excellent middlweight fighter who could fight with gloves or without, who fought against John L. Sullivan, and who helped train a lot of good fighters in his heyday (including Gentleman Jim Corbett). Here is his description for the "straight lead hand blow for the head": After judging the reach and quickness of your opponent, which can be best done by feinting, creep forward with both feet, and when an opening presents itself and you think you are within hitting distance, step forward with the left foot and instantly strike out straight from your shoulder with your left hand, hitting for your opponent's face, throwing the weight of the whole body into the blow. In striking this blow turn your head slightly to the right, to avoid your opponent's left-hand counter, or right-hand cross-counter. Avoid all chopping or striking with a downward motion, as such motions shorten your reach and lessen the force of your blow. Be careful not to get so far forward as to overbalance if you should misjudge distance. After delivering the blow, spring quickly back into position, guarding yourself at the same instant. This seems to signify that the lead hand straight is a power shot and not a light hit, at least according to his teachings. It's also interesting as you can see his bare knuckle background as he favors a vertical fist for his lead hand punch.
