The main reason why your money must be in a constant state of motion: the current employment of the monetary and fiscal policies of the Federal Government. The Government manipulates the "value" via these types of policies, the net effect of these moves is the depreciation of the currency. The constant movement of your money therefore is paramount.
Adam Smith even understood this:
"The increase of paper money, it has been said, by augmenting the quantity, and consequently diminishing the value of the whole currency, necessarily augments the money price of commodities. "~Adam Smith "Wealth of Nations"