Without fire the prairie would not have spread so far or prospered so well. Fire destroys the invading trees and shrubs. The prairie plants are not destroyed, their growing points and food storage parts are below ground. The plant's growing tips are protected because the soil is a good insulator and the intense heat does not penetrate beyond the surface. In contrast, the buds of woody species are exposed to the hot flames and usually will not survive.
Fire prevents prairies from choking on their own growth.
Without fire the dead leaves and stems accumulate through the years. This DUFF layer acts like a mulch and smothers new growth. The duff also holds nutrients, especially potash and phosphorus. Fire releases these nutrients for the plants to use again. The exception is nitrogen.