Why cut the top off a large tree to make a glade?

I think of the woodland as a whole community, trees are one part of this.  When the woodland canopy closes over it’s dark and open underneath, the ground-cover plants are sparse, there are few shrubs, not many seedling trees and an absence of decaying wood.  The fungi are not fruiting, it’s quiet, the birds are not singing and the insects are not flying.  There is need for a helping-hand by letting in some light, not too much, just enough for a shaft of sunlight to reach the forest floor.
We can either top one larger tree or cut a number of smaller ones.  The oak in this picture is next to another that dominates over it, so this one will never grow into a big tree.  It will however contribute a diversity of decaying wood:- in the trunk, the branches and under the bark. 
Many of the smaller trees are precious, such as birch, rowan and willow, and therefore retained.  Larger diameters of decaying wood are better for insects, which in turn provide food for woodpeckers and others. After topping the tree rain reaches the woodland floor and after only one year the ground plants are spreading.  The fungi will thrive in the damp conditions and in due course recycle the logs to provide nutrients for new trees to grow.  Thus the forest becomes alive and is being restored.