Competition for the canopy of sunlight is certainly a factor in both spreading of branches and tree height.
Trees don't live forever (although one wonders about those bristlecone pines estimated at 5 millennia years). Also, there are clonal or groups of trees that are actually one tree such as the 80.000-year-old Pando aspens!
A living tree can be a home for hundreds of species of insects. It can feed many birds. When a tree dies it leaves behind lots of nutrients for the soil and fungus and insects and birds.
Below are the trees in the setting sunlight of my front yard. It is exceptionally dramatic because the light bounces off the water from the river in the backyard and reflects on the trees.