Mourner’s Kaddish is recited after verses from Tanach that are supplemental to the main part of the prayer service. This was added to davening for mourners to fulfill their Kaddish obligation if they are not serving as the chazzan (the chazzan fulfills his obligation with the other Kaddish that are recited). For example, if a mourner is a child under Bar Mitzvah or if there are many mourners present (and since they cannot all serve as chazzan), the Mourner’s Kaddish is an opportunity for them to fulfill their obligation.