It mostly has to do with getting the most power to the ground.
When a car is turning, the outside of the car is supporting most of the weight. (Weight Transfer)
This means, in effect, that there is less traction available to the driving wheel on the inside of the car.
Without limited-slip, or in our case, a locked rear end; you soon reach a point where all the power you are applying is being used to spin the inside tire, not to accelerate you down the track.
There are some other things, but that is the most important part.
Does that help
Steve