Think of softness being the strings of a tennis racquet, imagine the racquet came with such loose string that you would struggle to hit a ball over the net, so you press the bat and tighten the strings, now it is able to hit the ball over the net but is still a bit slack and does not have the tautness needed for hard shots, so you knock it in and tighten the strings a bit more and it starts to hit the ball well and give the response you are after, now if you keep hitting it the face will compress beyond the perfect point and you will not be able to generate the same power as the spring has gone. Some bats never harden they just continue to compress with no real power.
Soft pressed bats need more knocking in, this is a real skill to get right and it not done well will lead to dull unresponsive bats.