Vdroop is there to lower the voltage to reduce power input into the chip to 'acceptable levels' under load as per Intels design specsheets. So when a MB manufacturer gets the specs for a new chip they are obliged to follow intels instructions.
I'm sure it also serves the pupose of prolonging the life of power MOSFETS on MB's too. But I think Stigma you may be asking the question... 'If the chips design voltage is within a certain voltage-current configuration, why don;t they just have no vdroop and just that lower V to start off with?' that i don't know!