What is the purpose of human existence?  Two circumstances will be considered: (1) if there is no God and (2) if there is a God.  Other views can be found of the Web (e.g., see Google: What is the purpose of existence?).

If There Is No God

Consider the result of the complete and utter destruction of humans.  What would be affected?  Maybe there would be some limited effect to the Earth.  But the Earth would not know or care.  It is difficult to think there would be any effect on the solar system or the universe.  If there were, it would surely be a very limited effect.  But the universe would not know or care anyway.  Humans would be gone.  So what?  Therefore, if there is no God, then humans are living pointless existences which will end at their death.

If There Is a God

Consider the result of the complete and utter destruction of God and humans.  What would be affected?  Again, the Earth would not know or care.  In this case the Creator of the universe would be gone.  So, the universe may be affected.  But, again, the universe would not know or care.  God and humans would be gone.  So what?  Therefore, if there is a God, then God and humans are living pointless existences.

Living a pointless existence seems a bad situation.  God provides a way to make the best of this bad situation by offering us a process that leads to eternal life, which is the kind of life that He lives.  The good part of eternal life is that it is, presumably, the best life that can be achieved.  The bad part of eternal life is that it extends our pointless existence into eternity.