We shouldn't. But Satan stirs us up to controversies and anger and debate.
"If God does exist or if doesn't, how does this affect our daily lives?" Mortal life might not be possible without God's intervention. If we truly depend on him for every breath we take, then his existence affects our lives enormously. Further, his inspiration and answers to prayer can guide us into wise and correct behaviors.
"This debate is just spiteful, and can only do more harm than good." You are absolutely right. Christians should not debate. We should simply tell the truth about our spiritual experiences, and if we have had none, we should seek the evidence for ourselves before trying to interest others.
"What do people ever benefit from winning an argument...?" They don't. Debate is not one of God's tools. Revelation is.
"Really, aren't we all like children bickering, squabbling and sometime fighting to some extent just over a simple statement to say who's right and what's wrong[?]" Very much so at times.