We're dancing around the real reason here. Accurate shooting demands things be done the same way, every time. A straight wrist stock is difficult to get the same grip on the stock every time. Having a "finger rest" forces the hand to be in the same position for each shot. Take a look at the Scheutzen style rifles and the elaborate lengths they go to achieve the same thing. For many rifles sold to the general public, it was a marketing ploy by having features known to be effective on target rifles of the era put on average rifles, so then in the mind of the consumer, the average rifle should be just as accurate. Marketing is marketing then or now.
How does it work? By the hand not being in the same place, trigger control varies and by "reaching" with the finger to the trigger, the gun can be canted (tilted) during the shot leading to inconsistent accuracy. It can also lead to "pulling" the shot to the right for right hand shooters. It can also lead to shots being low to the right or high to the left. Accurate, consistent shooting requires variable control, there is no other way.
If you think I'm kidding, deep dive into Scheutzen and competition rifles of the day and then do the same for trigger control in match shooting. You'll find hand position is very important and anything inside the rules that helps a competitor is a bonus and certain to be on the rifle. Most folks outside the competition shooting world have no idea what a serious competitor will look at to eliminate variation and improve accuracy. So basically, nothing in the competition world has changed, just the powder and the guns.
