When you play ping pong or table tennis, you are not as concerned about how the ball deforms, as you are about how far it travels and with what velocity it lands on the table. In other words, you are more interested in the rigid body motion of the ball, rather than its deformation.
Consider a robot arm used in an electronics manufacturing facility. In this case, we are interested in simulating the complex motion of the arm when one or more of its actuators are activated. The development of internal stresses are secondary concern.
When we are more interested in simulating the rigid body motion of an object, rather than its deformation, using rigid body dynamics is the fastest and computationally most efficient way.