If you burn the forest often, it doesn't build up a significant base of fuel. Basically what makes forest fires destructive and vast is that all the stuff fallen and dead on the ground is burning hot and long enough for the live trees to ignite. By burning the forest over to get rid of deadfall every couple years, the fuel load on the ground never gets deep enough to threaten the trees, and the saplings are knocked back. The overall effect is that you get a very open forest with significant sunlight on the ground, promoting low vegetation and ideal game habitat. They did burn the forest for agriculture too sometimes, but that's not the only reason. We don't use burning nowadays, instead we go in and selectively cut, taking out the trees that are too crowded or too weak to allow stronger trees to thrive and to open the floor.