Same principle as starting out on the track on street tires. It forces you to use the brakes correctly, and have a good line. You can run the same times with sticky tires by driving sloppy and using the tires to cheat you back on line, but all the tire has allowed you to do is equal a street tire by driving like crap instead of learning how to handle the car. It's like electric nannies, if you're not driving the tires anywhere near their limits, you're really not driving the car, the car is driving you.
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