Well primarily because they are dead, they become dry and brittle and have a tendency to break off in strong winds. Especially the winds of our Nor’easters in the NorthEast. Also dead stubs and branches can be entryways for fungi and bacteria to enter live healthy wood at the attachment points and spread to other parts of the tree.This is especially a concern if the tree has an aggressive or infectious pest attacking it and/or trees nearby. Lastly, on some species, pruning out dead also will greatly enhance a tidy appearance of your trees, giving them a less ragged and messy appearance.
When weighing whether or not to spend the money to have them pruned out, some Q’s to ask yourself are;