Over time, old forests build up with dead trees, decaying plant matter, and old vegetation that chokes out and prevents new growth.
When a forest fire breaks out, it destroys all these things and returns them (and the nutrients they’ve been harboring) back to the soil in the form of ashes. The underbrush is cleared so sunlight can reach the forest floor and new growth can emerge. Invasive weeds are gone, insects are eliminated, and newer, younger growth explodes.
I have to say, I’m not totally opposed to it.