Why is there a need to ground a circuit?
Electric circuits need to be closed. The hydraulic analogy might help you in this sense: suppose you have a garden fountain and want to recycle its water;  if you want water to circulate by means of a pump, you need a way to put the water back in the pump. That's what the ground connection is for.

Sometimes you have closed circuits using other wires, and a ground connection (or a shorter, direct ground connection) might be required to keep at bay the effect of EMI (electro-magnetic interference). 
For example, this is the output of an Arduino Nano sensed by connecting only the tip of the oscillocope probe to pin 13.



And this is the same output when I connected the ground tip to Arduino's GND (actually to the USB frame):