Why do helicopters need tail rotors?

When helicopters main rotors are turning, it is the natural reaction of the fuselage of the helicopter to spin the other way, this is known as torque, a tail rotor spinning counteracts this force by pushing the other way, The rotor pedals change the pitch of the blades on the tail rotor, if the pilot wants to spin left, he compresses the left anti-torque (yaw) pedal, and the blades will alter their pitch to offer less resistance, making the fuselage spin left, if he wants to turn right, then the pilot will press the right pedal,this will make the blades on the tail rotor change pitch to make even more resistance...this overpowers the torque force and makes the helicopter turn right. hope this helps :)