Have you ever tried to accelerate quickly to merge onto a high-speed highway in bad weather and been stuck spinning one wheel? Open differentials are important when making turns as they allow for the outside tire to move faster than the inside tire; unfortunately, they also allow all the power to go to the wheel with the least amount of traction. In most driving conditions this is fine, but when driving in inclement weather or on rough or muddy terrain, the vehicle can get stuck spinning one tire on a slick spot while the other three have great traction. Hauling or towing a heavy load can make driving in these conditions with an open differential even more difficult.
To combat this, many manufacturers offer some form of traction aid. These range from applying the brakes on the spinning wheel(s) by using the antilock braking system to locking the differential so that both wheels spin at the same speed regardless of what surface the tires are on. Many systems offer a combination such as the Toyota Tacoma, which has active traction control and an electronic locking rear differential.