Why is hanging drop preparation visualised at the edge of the drop?

Question

Why is hanging drop preparation visualised at the edge of the drop?

Answer 1

A couple of reasons is that the concentration of oxygen will be highest at the thinnest portion of the drop.  Bacteria will often be more active and concentrated near the edge.  

The second is that it is easier to locate the interface between the air and edge of the droplet!  Once you've located the edge, you can move the slide to see other portions of the droplet.