Here, on our planet, all organisms breathe in oxygen and breathe out carbon dioxide. Humans need an atmosphere with a certain amount of each oxygen and carbon dioxide for respiration. On our world plants produce oxygen as a byproduct of photosynthesis.
On another world a different mechanism could evolve, and respiration would not necessarily involve oxygen. Here life is carbon based, oxygen breathing and DNA defined; a different world would have an entirely different evolutionary path and while outcomes may be similar biology could be very different.