The answer is simple. Dogs bark because they are telling you something.
Barks can be divided into different types based on context:

Continuous fast barking
 – Alarm Bark. Problems - someone is entering our territory.
Fast barking with pauses
 – Warning Bark. Asking you to investigate.
Short bark:
 – Surprise Bark. If it's repeated: "Look at this!"
Faltering bark:
 – Want to play.