They need the sunlight to prevent organic matter on the ground to dry and not decay in a way that could cause rot. Trees also need to prevent infection from ot

They need the sunlight to prevent organic matter on the ground to dry and not decay in a way that could cause rot. Trees also need to prevent infection from other trees, need to prevent damage from colliding from other trees, and mass damping doesn't work if trees are touching. But they also want to be close in order to share nutrients through their roots. There are many reasons why trees would want to do this, from an evolutionary standpoint.