The average person is not trained to spot a tree that is in distress. They have the experience and knowledge to tell if a tree is a danger and if it can be saved or not.
A regular inspection of the trees will make sure they are not a hazard to people or the property. A professional tree service will look at the overall health of the tree. They know the different species of trees and what they should be on the lookout for. For example a maple will form unions of weak branches that will break during a storm. The professional tree removal expert will also take into consideration the age of the tree and the size.
A tree inspection can find more than just rotting wood. They can find issues from the group up. If the twigs are forming a crown it can be a sign that there is damage in the roots of the tree.
If the tree is decaying it should be removed right away. The tree can fall over at any time.
If there are co dominated stems in the crown this can be a sign of a hazard. Rot can be hidden between the stems and a big storm can snap the tree.