Because modified duration can effectively measure the impact of changes in interest rates on price changes in fixed-income securities.

Modified duration is calculated based on the duration of the bond, but the calculation also takes into account the impact of the bond's maturity, interest rate, and the bond's present value and future cash flows.

The longer the modified duration, the more sensitive the bond price is to changes in interest rates.

In practice, investors often use modified duration to evaluate price changes in fixed income securities in order to better manage their portfolio of fixed income securities.