Racing cars go very, very fast.
Speed is the point, speed is fun, but bad things happen quickly when you're going really fast. Sometimes the car rolls over. Window nets are required to protect the driver when that happens.
Before the 1960's (and sometimes after) many drivers hoped to be thrown out of the car in the event of a rollover or hard impact. American driver Masten Gregory made quite a reputation-- and a few surgeon's portfolios--performing high speed dives from out of control cars. When the great American driver Dan Gurney won the Belgian Grand Prix at Spa-Franchancorps in 1967 the car carried no seat belts so he would not be burned to death. You see, Formula 1 cars of that era were made so that the driver's seat was structurally part of the gas tank.
The problem with that idea is that when people are thrown from a car, they retain all or most of their momentum. The ground is hard. Getting thrown from the car may keep you from getting burned, but it can paralyze, crush or decapitate a driver due to contact deceleration.
Wile E. Coyote survives such things. People don't do so well.
So the trend in race cars became to keep the driver in the car, where he or she can be protected. Fuel cells eliminated almost all danger of fire. Roll bars came first, and the hoops are retained for most open topped race cars. Roll cages first appeared in funny cars, but quickly spread to road racing and NASCAR, particularly as the cage structurally reinforced the car. Five point harnesses became the minimum permitted by the GCR.
That protected the head and body of the driver, but what about the arms?
Imagine what happens during a rollover. Usually the car will roll longitudinally, if for no other reason than the car is a lot rounder to the sides than front and back. The driver is belted in to his braced racing seat so his body is going nowhere. With a HANS Device or even a neck brace, head movement is often controlled. But the limbs must remain movable if the car is to be driven.
Feet aren't much problem, as they are tucked down under the dash hoops, and between the front pillars of the roll cage. Plus, legs don't have anywhere near the range of motion allowed arms. Keeping your hands on the wheel might protect the hands. But that's hard to do, as inertia often rips them from the wheel. And if you fear a hard impact, it's not a good idea to keep them on the wheel. A hard impact on the steering rack can twist the wheel hard enough to break your thumbs.
This means inertia may control the drivers arms during a rollover. The arms literally flop around. The cage is strong enough to protect you, but if any part of the driver exits the box defined by the roll cage's bars that limb may be crushed or amputated if it gets caught between bar and ground.
Window nets were developed to prevent this. They are webs of nylon, square or of a fine mesh, and are fire resistant. The GCR requires that the net be attached to the roll cage itself, so arms don't get crushed between body and cage. They prevent the driver's arms from coming out in the event of an impact.
If you drive an open roof car, such as a convertible or a formula vee window nets are not possible. Therefore the GCR does not require them. Instead, it requires wrist restraints to hold the driver's arms tight inside the car. Ideally they should be strapped to the forearm, but the actual attachment point to the driver is undefined, so they often get up on the bicep where their value is questionable. Grid workers are required to inspect the driver's belts and nets before the cars go out. Corner workers are instructed to call in loose or absent nets, and a missing net can result in a black flag.
The GCR requires that window nets be inspected during a cars's annual tech, and replaced every five years, like driver belts, so they do not fall prey to sunlight damage or are weakened through aging.
the ideal steering position for racing is to hold your hands at three and nine o'clock, fingers around the wheel, but the thumbs pointing at 12 o'clock. Thats enough to control any car with power steering, which is also common on race cars. But your thumbs will not break.