Thinning is important because it adds value to land. It does so by increasing forest health, opening up wildlife habitat, managing for future timber harvest, and last but not least it mitigates wildfire damage.
Thinning is a general concept that covers a number of different prescriptions. The general concept is that not all trees are taken and removed, rather only some are taken and some are left. But which ones to take, and which to leave?
Selective thinning in a mixed canopy situation takes some trees that are or will soon be at the end of their life. It also takes some regrowth, but at the same time makes sure to leave varying age classes of trees. Culling of unhealthy/diseased trees is another example of thinning, with obvious benefits. Dense stands of new regrowth are not natural and occur in places where timber has already been worked, often in old clear cuts. Thinning this regrowth so that proper distance (figured by tree species and climate) exists between trees results in trees that are able to grow, rather than a tangled and stunted forest.