Torque increases as revs increase from idle to a certain figure and then falls as the revs increase above this figure. Acceleration is proportional to the amount of force pushing the vehicle forward, so maximum acceleration in a given gear is attained when maximum torque is attained. Power is force multiplied by speed, and maximum acceleration is attained by having maximum propulsive force at the wheels. Maximum acceleration at a given speed is attained by having the engine operating at maximum power. Driving a car is easier and more relaxed if it has a flexible engine.
Flexibility even becomes a safety issue in four wheel drives.
Conversely, if it speeds up by a small amount, torque will tend to decrease and discourage further increase in speed. The driver can compensate by opening the throttle. Conversely, if the speed increases then the torque increases so the speed increases more. The driver has to actively compensate for these variations so the car is less relaxing to drive. The driver cannot correct for a fall in engine revs and loss of torque if the throttle is already wide open, except by changing into a lower gear.