For a time, this was just how most boxers threw the rear cross or straight. Though it's hard to call it "straight" since the punch is often crooked. That's because how the fist traveled depended on the location of the target rather than lining up the target to the puncher's predesignated flight path.
This method doesn't necessarily maximize reach, but it does keep your defenses up and hides your rear punch until the last second. It also moves your head out of the way.
The way people used to throw the rear cross isn't necessarily "right," and the way people throw the straight now isn't necessarily "wrong," or vice versa. But it is helpful to study the past and glean lessons from it. Many people who say, "This is how it's always been done," never look at the evidence because boxing, like all combat sports, has changed, and what people think is tradition is often an adaptation to the conditions (pleasing styles for PPV).
Under the Liberation Martial Arts model, we understand you can never throw the same punch or strike twice. What is executed should be a reaction to the opportunities presented. So rather than fixating on one way of doing things, one of the takeaways from old-school boxers should be to adapt a technique based on the conditions. Stares in Marx.
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Tape Study: Old School Rear Cross