When a JIVA commits sinful activities and builds up the papa karma, eventually suffering is inflicted upon the JIVA when the fruits of the sins mature. After undergoing the suffering either in the Human body or the Yaatana sharira in the hellish planets, the JIVA is given forgetfulness of the past suffering, I the JIVA could have remembered the previous suffering in its new life he would dare commit the same sinful activity once again. The prime goal of suffering is to purify oneself and to rectify past sinful tendency how forgetful given by the lord play a role in the well being of the JIVA.
Hare Krishna, All glories to Srila Prabhupada… As described in the Srimad-Bhagavatam towards the end of the 5th canto, the various descriptions of hellish planets that are part of this material creation, the living entities going through many of them for many many years, if the forgetfulness was not given, then it would have been impossible to lead a normal life. Therefore slow there is evolution in the consciousness and in the human form of life there is full capability of using the “free will” given to us by the Lord. Only by using our free will we can get out of this cycle of birth and death and attain our constitutional position. We need to constantly practice this.