It seems to me that the main purpose of our time here on earth is to prepare us for eternity. No one is promised a good time here on earth... in fact we are promised troubles.
But the Bible speaks of God testing us and purifying us. To what end? The next life is the pleasurable and fulfilling one, not this. But God wants people who share His Spirit and His will.
We know this earth will be destroyed, as will these bodies we currently have. What survives is our spirits... that is what God is developing. And for some of us, no doubt, He can develop us best through physical pain and disability. It didn’t have to be this way, but we transgressed. Still, for us Christians, this world is the closest thing to hell we will ever know. And thank the Lord for that.
I’m reminded of something CS Lewis wrote... I forget where he wrote it, but he said that we are all affected by these bodies we are stuck in; we will not really see each other for who we truly are until we are free of them. And at that time, there will be surprises.