The general idea is that your power comes from your connection to the ground.
So when do I need to be touching the ground to benefit from it?
If you jump and punch you can still move your hands quickly - so it isn't the need to generate speed (the start of the punch) that requires this connection.
But on impact I want that speed to be connected to as much mass as possible -> to get my whole body and legs involved. And it is at this point the contact with the ground is important.
With that said - it is very difficult to time just this part of the technique within a step/movement - hence you often see boxers "plant their feet" when they are trying to hurt their opponent, the punches will be more powerful like this, but they are less mobile/prone to being countered and less able to adjust distance if the opponent is moving.
You also often see this being ignored with the jab - many fighters use this as a distraction/nuisance and distance gauge to bring the back hand (or legs/other tools depending on discipline) in to play. A few boxers do employ this timing extremely well and will break opponents down with the jab - mainly because the opponent is used to fighting a jab that has no power. If you can get this timing right - it is definitely a useful skill to have/could be the difference in a bout.