The story of the great flood was told as many other stories of the OT to enhance the power
of the god of the ancient Hebrews. One could argue extensively on the reason why would
god do such a thing, the bigger argument really comes from biological reasons and
rationalized physics . Any occurrence outside of the natural laws of psychics, moves into
a faith believing miracle state, where anything imaginable can happen.
Were the ancients compelled to lie in telling stories of their gods to create consuming
relevance, yes they did for the greater the god, the greater and more established relevance
was created toward themselves (priests, seers) who were said to be connected to these gods.