There are several reasons for planting trees, some mutually incompatible.  In past centuries woods were established primarily for sporting purposes, hunting and shooting, which is why we have so many small farm woodlands.  These mixed species woods with glades and rides are also good for wildlife, another reason for planting.  Then there are forests established for timber, for furniture, construction, fencing, pallets and fire wood.  Here the density needs to be greater to ensure tall straight growth with fewer side branches.  Whilst hardwoods are planted, the majority is conifer because it is quicker growing.  Much of the expansion of forest established by the Forestry Commission over the past century has been of Sitka spruce, but a dense plantation of conifer for timber production provides very little habitat for wildlife.