The fire of Gehenna is in fact an act of kindness that is performed for the soul so that it will be able to receive a proper reward:

Even when [a soul] enters Gehenna and the fire purifies it, this is in order to cleanse and purify…. The Holy One, blessed be He, acts kindly toward the soul, washing away its stains in order to give it a proper reward. This is like a mother who, by washing off her child’s dirt, is acting kindly toward him. If she leaves him in his filth, this is cruelty, as his body [his health] will deteriorate. Even though the child cries and is miserable, it is all for his own good. (Rabbi Eliyahu de Vidas, Reshit Ĥokhma, Sha’ar HaTeshuva 3:38)

The afflictions of Gehenna are intended to rectify the soul so that it can ascend to the Garden of Eden:

It was said with regard to Aher, “It is better that he be judged and [then] be brought into the World to Come [the Garden of Eden]” (Ĥagiga 15b). All the suffering of Gehenna is worthwhile so that the soul can take pleasure in the Garden of Eden. It is impossible for it to attain this delight until it has been cleansed by means of some amount of suffering. (Rabbi Shneur Zalman of Liadi, Torah Or, Exodus 73:2)

Gehenna achieves purification of the soul. Its purpose is to cleanse the soul from the evil that clings to it:

The purpose of Gehenna is to purify the soul from the filth of evil within it…. For the soul to receive the light of heavenly pleasure [in the Garden of Eden] and bask [in the radiance of the Shekhina], it first needs to undergo purification in the fire of Gehenna, which separates the evil from the good. (Tzemaĥ Tzedek, Or HaTorah, Exodus 11)

Further reading: Read about the life and death of Aher in A Concise Guide to the Sages, p. 339.