Having just returned from Yosemite, I noticed vast areas of blackened trees and the skeletons of dead trees. In between these dead trees were new trees cropping up. (It looked like a checkerboard - alternating dead trees and live trees.) The dead trees will eventually decompose, leaving behind the new growth (which will then be much taller). That's the reason - to save the new forest, which is in the process of growing.
But in cases where a forest burns totally to the ground is there any reason that land is more suitable for a forest than any other land?
I guess my real point is that if liberals feared that forest fires would force them to surrender "their" protected lands to developers, they might take more of an interest in stopping forest fires.