Answer:
Financial investors earn money through the purchase of bonds, be these public (of countries or states) or private (that is, of companies) through speculation regarding the rise or fall of their value with respect to the purchase value, as well as with the interest that these bonds accrue periodically.
Thus, if the bond rises in value due to a good economic performance by the issuer, investors can sell it and make a profit, or even buy bonds at times of low values, expecting increases.