One thing that I think needs to be clear is that God created us, and He also created time. The “after life” cannot be fully grasped in any number of ways, but one is that there is no time there.
The details, quite honestly, are very sketchy as to the purpose. But one thing that I believe is quiote commonly held: everything on earth that is good is a foretaste of heaven. Imagine anything that is beautiful, tasteful, joyous, etc. on earth, and multiply these things greatly - and that is heaven.
Certainly, we will share in the beatific vision of God. For those who do not believe in God, you may say “big deal.” But think outside the box. If there is a God who is awesome enough to create everything, all of us, how awesome must He be in appearance? How awesome must sharing heaven with Him be? So, if indeed we are correct, then this reward should be able to appreciated for even those who do not comprehend or believe in God.
If one is to believe many of the visions of heaven, hell, and purgatory that past saints have had, then it is evident that heaven is the complete outpouring of all things good to all of us, and hell is the complete absence of those things. Even if one doesn’t believe in actual “fire and brimstone” in hell, the fact is that anyone who chooses to live without God in this life has opted to live without God for eternity.
Since God still reaches out on earth, he can still bring certain happiness and joy to anyone - including atheists. The atheist may not recognize that any good thing on earth comes from God, but it does. However, in hell, God allows the dignity of that choice to come to full fruition. In hell, there will be no presence of God. Without God, there can be no light, no happiness, no joy, no love.
As for the ultimate purpose God has in store for all of us for eternity? I’m sure there are many. But just as I wouldn’t have been able to tell you what my purpose was on earth while I was in the womb, I can’t say what all is in store for us.