Im just learning the basics of AE and Im just trying to get my head round this particualar aspect of it. Im going to say what I think is correct and if its not then please do correct me:
Theres two aspects you control. The point of interest which controls what the camera is looking at and moving the camera itself which changes the position of the camera.
A null object is an invisible object/layer.
Parenting a Null object to the camera
When you parent a null object with a camera you are just connecting them. This allows you to control both due to you controlling the null object which in turn controls the camera. Essentially what the null object does the camera does as well.