Fire burned away all the built up debris on the forest floor and allowed sunlight to reach the ground. The increased sunlight on the forest floor created an opportunity for other plants to grow - plants which provided food for the Native Americans and shelter for wildlife. This cleared ground also gave redwood seeds a place to grow. Second, fire leaves ash, which contains a high concentration of minerals and nutrients that are leached into the soil by the rain. These nutrients help other plants grow.
Fire also hollows out the inside of many trees, leaving behind cavernous holes.
This does not kill the tree, since the major part burned is the heartwood, the dead wood inside a tree. The only living part of the wood is the cambium, a thin layer between the bark and the heartwood where all growth takes place. This is why trees have growth rings. Each year, a new layer of cells grows and produces a growth ring. Redwood bark is thick, up to 12 inches on some older trees.