Well, the title sayes it all... i think.
Why should i use the middle pin (+5V) on KY-008?
It's looks like almost every one keep saying "you do not need to use that". But why? Why is it there when we do nott need to use it?
Somebody? 
Impatient are we?
Well, these three pin modules of which the KY-008 is an example, are intended to use the wiring convention of servo motors with the positive supply on the centre pin (red), negative on one side (brown) and control wire on the other (orange). This allows them to be used with a "servo shield" or "sensor shield" and the readily available matching "Dupont" cables.
In the case of the KY-008, the 10k "keep-alive" resistor will cause a very slight glow but no laser action. Is it useful for anything? Not really obvious. 
The laser diode with its 91 Ohm resistor will draw about 25 mA so it can be driven directly by an Arduino (ATmega) pin.