mdd1, why do people go through trials and suffering in this life too? Maybe Hashem should’ve made a pain-free existence for all living creatures? We can grow from suffering and become better people, that’s why. Having a pain-free existence means would mean that we would stay stagnent and never grow spiritually. Same in the spiritual dimension, the neshoma goes through Gehenim to become a stronger and better spiritual being to be able to enter Gan Eden.
… Actually that’s my guess about Gehenim. It is probably only part of the real reason ( reality can have many aspects or truths to it, not only one) or not the real reason at all. Only Hashem has all the answers to the mysteries of creations.