Dents in pizza dough
Bread dough experiences "oven spring". That is, the water in the dough turns to steam and gives some fast, extra lift when the dough is first placed in a hot oven.

For most breads oven spring is a very good thing. For thick, flat breads like focaccia and pizza it is not good thing. The spring, or any other rising, will make the dough no longer a flat bread.

Putting the dents in flat breads is simply to help keep them flat when they bake. Unlike in pastry dough, where the docking pierces the dough and lets steam escape, the docking in bread dough shouldn't cut through layers but simply leave indentations.

Pizza doesn't always need to be docked because the weight of the toppings is sufficient to keep it flat. I don't dock my pizza dough. 

The indentations are also nice because they increase surface area for toasting and hold on to toppings, such as the olive oil on focaccia, but I don't believe that is their primary purpose.