Q:

Equilibrium of a rigid body

Why is it necessary for a body to have the net torque acting on it be balanced along with the forces for it to be in equilibrium? Isn't torque just some special case of force like rotation is a special case of translation? Why doesn't it suffice to have only the forces balanced for the body to be in equilibrium? I know there are examples which prove otherwise, but I need a more theoretical explanation. The case where the net torque is zero even when the force is not is understandable, since torque only represents the rotary component of force.

A:

There would have been no need of introducing the concept of torque if we were only concerned with motion of a point particle (as the name suggests, it is hypothetical physical object having mass but no size). There is no concept of rotation of a particle and thus no need to introduce the concept of torque.
The concept of torque arises when we are concerned with system of particles. Here, there is a possibility that the net force on our system is zero but still the body is somehow moving. This possibility arises because the forces may act on different particles and there may exist internal interactions as well,causing the body to rotate if it is rigid.
That is how the introduction of the concept of torque is justified. 
Torque is qualitatively a twisting force. Therefore, if net torque is zero, then the body cannot rotate. And if net force is zero the body will not translate. That is why for mechanical equilibrium, both the conditions must hold .
I hope I made myself clear. 

