The pressure of walking is much higher than most people realize. In the case of women's high heels, it can momentarily be hundreds of pounds per square foot. Without a good quality cushion, this is tough on the carpet and tough on your feet. This can help reduce the grinding action of embedded dirt that can cut and fray carpet fiber. The most important thing you can do to prolong the life of your carpet is to give it a good vacuum cleaning often.
Dirt grinds down carpet fiber the way sandpaper wears down wood, and thus shortens carpet life.
A powerful vacuum is a carpet's best friend. When you vacuum, good quality cushion lets more air come up through the carpet, carrying more dirt along with it. The result is more effective cleaning every time. It can reduce carpet noise A carpeted environment is quiet, but a carpet installed over cushion can reduce noise even more. In tests comparing cushioned carpet to uncushioned carpet, cushioned carpet significantly reduces noise.