I need help with identifying this, and I'll have a lot more questions.
The hole on the back of the figures is mostly used for backpacks and other accessories as shown here in the figure on the left:
In addition, it is also used sometimes to secure the figures into position. For example, in some open cockpit vehicles there is a peg at the back of the seats, that the hole on the back of the figures can attach to. This keeps the figures more secure in their position.