Hi stefan,
Nice question! That is a connector I made soldering a 3 male pin connector (like this one) to a piece of universal PCB (like this one). Of course, I soldered 3 wires to the terminals on the back of the board that I ran all the way down the arm to connect to the board at the base.
It serves as a connection interface to plug any kind of actuator that needs (VCC, GND and Signal).
Hope it helps!