If there is no Heaven or Hell, if there is no God, if nothing remains after Death and there is no state of being  and absolutely nothing after death, if this is just one Life and one chance to do whatever we desire 
Then  :
Why will human beings be on the side of virtue, be good  , in the absense of  Law and Order ?   In other words :  Why will someone not loot, rape and kill for his own pleasure, in the absence of – Police, Court, fear of God  and his own conscious  ?

The question itself though establishes the essentiality of Concept of God in the mind of the mass, but  answering  the question with as much rationality as is possible, I will say :

You know that good deed makes your Life and life of others beautiful and you also know that given a choice YOU WILL NOT SEAT BY A TRASH BIN INSTEAD OF A BEAUTIFUL GARDEN !  So that’s that !!