Most (if not all) of the time it is in refutation of statements from the fairytale believers who are peddling their lies willy nilly. It is because people generally were too ignorant to perform such refutations in the past that these myths have gained such credence.
If people want to believe in God/Unicorns, I agree that it is entirely a matter for their own imagination.
It is when they 'insist' that others collude with them in this delusion and try to foist the proscriptions and bizarre tenets of *faith* upon the rest of the population via Legislation that I start to bridle.
What is it about believers that they spend so much time and energy trying to foist their views on the atheists? Probably because the very few that do, on both sides, enjoy a good debate.