There is a difference between making up HP before the game and then sticking to it and deciding mid combat that the PCs are not doing well enough in the fight (or doing too well by killing the enemies too fast) and arbitrarily decide that the combat needs to be made longer or shorter by adjusting HP.
In the latter case where the DM decides anyway on the spot when monsters die or not its easier to not have HP at all. The result is the same as the combat takes exactly as long as the DM wants it to be and ends with the result the DM intended. Abolishing HP is just the more honest way for the DM to ensure it (and also removes preparation time).
I have no great love for this technique and no intention to pretend otherwise.
No matter the reason, be it the BBEG dying too fast, the encounter being harder than expected or just not memorable enough, changing HP is just a way for the DM to lengthen or shorten an encounter to what he wants it to be or ensuring that the outcome is what he wants. And if that is his goal, to control the flow of combat so that it has the intended (dramatic) effect instead of letting the dice fall where they will, why fiddle around with HP instead of just deciding when a monster dies?
Because then the damage, or rather the whole fighting ability, of the PC does not matter any more (in a game where a big part of the rules is about fighting)? Do they matter when you decide after a first round of good rolls from the players that the monster they are fighting has twice the HP so that it can have the intended dramatic effect?