In karate, why is it advantageous to apply a force for a short time?

From momentum perspective,

F = ∆p / t

Where p is momentum

So, the force is inversely proportional to the time taken, so the shorter the time of impact, the higher the force impact.

So, it is advantageous to apply force for a short time and the major advantage is that the punch will have a heavy impact on the opponent and with that he might knockdown his opponent.