While catching a fast moving cricket ball, a fielder in the ground gradually pulls his hand backward with the moving ball, In doing so, the fielder increases the time during which the high velocity of the moving ball decrease to zero. Thus, the acceleration of the ball is decreased and therefore the impact of catching the fast moving ball is also reduced.
If the ball stopped suddenly then its high velocity is decreased to zero in a very short interval of time.
Thus, the rate of change of momentum of the ball will be large. Therefore, a large force would have to be applied for holding the catch that may hurt the palm of the fielder.