This equation is always true by definition. If an economy has $5.00 of money, and each dollar is spent four times a month, the total monthly spending must be $20.00.
This equation is useful if we assume that velocity is constant. In that case, any change in the money supply will lead to a change in nominal GDP (product of P*Y)
So if the level of M goes up by $100, according to the equation of exchange, the product of P*Y must increase by $100.