I have a KY-006 buzzer at home which works fine. I would like to solder it off the PCB and put it on my own board. I still couldn't figure out the following though:
Usually you leave the VCC pin (middle pin) free. I found some wiring plots though especially for raspberry where this pin is connected though. Why is this? What does this do?
How is the wiring on the pcb itself? (can not see it properly) It looks like the buzzer has only 2 pins. On the PCB are no other electronic parts, so it simply does not make much sense to me. Maybe I have overlooked something?
From what I can find of the KY-006 module, Vcc pin is not connected to anything. You should be able to unsolder the buzzer and put it on your board. Just connect the ground and the signal (+) terminals.
The actual piezo has only two pins and there is no other component on the PCB so indeed the centre - VCC - pin is not connected. There are also a couple of different versions of the PCB which is clearly also designed to accommodate two different purposes.
The three pin arrangement is purely to facilitate a modular system where different modules use a system common to servos with readily available three wire cables to connect to "expansion shields". Note that there are two alternate (incompatible) pin arrangements for servo connectors.
Possibly the Raspberry Pi arrangement you cite is using a corresponding expansion shield.