Nothing better to do with it. Sometimes you cannot spend it fast enough and you are forced to save. For emergencies and unforeseen expenses and for something you want to buy later. Saving money is not important if there are things you want to buy. Opportunity cost is what you pay when you buy one opportunity.
Opportunity cost is an economic term for paying too much for something.
Opportunity cost is a term the government uses when it spends too much money. Opportunity cost is the cost of passing up an investment that would pay more than the one selected. Bank savings account. U.S. savings bonds. Stock market or mutual funds.