From my economics text book (coauthored by Ben Bernanke):
"Store of Value
As a store of value, money is a way of holding wealth. An extreme example is a miser who keeps his life's savings in cash under his mattress. But even someone who spends his cash wages 15 minutes after is using money as a store of value for that short period.
In most cases only money functions as a medium of exchange or a unit of account, but any asset-for example, stocks, bonds, or real estate- can be a store of value. AS these other types of assets normally pay the holder a higher return than money does, why do people use more as a store of value? The answer is that money's usefulness as a medium of exchange makes it worthwhile to hold, even though its return is relatively low."