As trees grow, their structure or "shape" changes. When limbs and branches dry out and die, they eventually will erode the entire trunk, thus shortening the lifespan of your trees. Dry or dead wood is also a breeding ground for insects and disease, not to mention a potential hazard, even dangerous.
Under these conditions, timely pruning is necessary if you want your trees to live a longer, fuller and healthier life. Also, pruning your trees will provide safer conditions and be less expensive in the long run.