Answer:
Ok i have a really weird theory.
Oxygen actually kills us we just don't know it yet. We need it too live but something in it also poisons us and causes us to die in around 100 years from birth. that's why we all die
real answer
we need it to breathe, when we breath we inhale in oxygen and release carbon dioxide which allows trees to grow and they produce more oxygen. We cant rely on anything else because oxygen is important and without it we die. Oxygen is one of the purist airs.