This helps in bringing back the yields of old bonds in - line with the freshly issued bonds. (money.stackexchange.com)
Indeed, if rates rise gradually, the interest payments on a bond fund will increase as older bonds mature and newer ones are purchased with higher coupons. (moneysense.ca)
Of course, the opposite is also true: if interest rates fall, older bonds with higher coupons become more valuable. (moneysense.ca)