1) The primary goal is to locate and eat insects, as well as their potential eggs and larvae. Many invertebrates lurk behind the bark of a tree and call it home, hence more food for the woodpeckers.
2) Woodpeckers will also fully take advantage of already dead or dying trees to hollow out a nesting cavity. This is because they are primary cavity nesters. This simply means that they will hollow out their own nesting cavity opposed to using an existing one. It is not uncommon for these nesting cavities to be utilized by secondary cavity nesters of other bird species such as nuthatches, blue birds, and even small owl species such as the Eastern Screech Owl. These cavities may also be utilized by mice and squirrels. Making these holes a critical habitat booster.
3) While humans may use posted signs to mark boundaries, woodpeckers establish their territories with the deliberate loud pecking. This typically coincides with them seeking mates. Often referred to as "drumming", male woodpeckers will try and make the loudest sound they can to establish themselves and attract a mate. This is often what they are doing when they are pecking at metal or other hard objects rapidly and loudly.