The most basic purpose of life is to pass on your genes to the next generation (or not).
The purpose of any lifeform is to live and see their offspring live.
That is how life sustains itself.
You can make your life useful to others, you can enjoy it. You might as well. You only got one life after all.
whether you believe in the existence of heaven and hell?
The nature does not care if you believe in hell or heaven. Earthquakes and hurricanes happen no matter what you believe.
Species survive or go extinct no matter if you believe in Ra, Zeus or more recently in Jesus.
what you ever have a problem? how do you solve the problem? Do you not care with the problems you are currently facing? problem is a test, so earthquakes etc it was is a test. then why there Mosque, Church, Temple, Shrine, Pagoda, Synagogue. What are they doing there? what they expect there? if they are not expecting paradise? then why there a place of worship if you do not believe in the existence of heaven and hell? Do you know the "faith"?
If you have a problem, you use that thing between your eyes to solve it.
People came up with the concept of 'God(s)' to fill up the gaps in the understanding of how the nature works.
This need to have answers to everything is well ingraied in all of us.
Some of us fill this need with imaginary concepts, others sit down and try to investigate, reason out the solutions.
The sad reality is that there is no heaven or hell. I wish they were. Some of the fucked up Muslim followers (ISIS or Sharia Law sympatizers) or Christian pedophiles should definetly go to hell. But unfortunately we all die and decompose, whether you are a saint or an ISIS executioner.