The zoom feature will allow you to define link areas with greater accuracy. For instance, what if we wanted to make a link area of a face? In "normal" view, we define our area like this:
But if we use the button to zoom in on the face, we see the area is not quite perfect:
So lets zoom in even further, then re-define the area.
As you can see, the farther you zoom in, the greater the accuracy you can achieve!