It separates (theoretically, as it can't be measured) the price change due to faith-in-currency from price change due to market forces on the products.

That is, the price of good may fluctuate as function of how much current labor you need to buy it, or as a function of how much of your past savings it will cost. The diference between those measurements is due to money supply (in part, for there are other complicating factors)