Yup, while free atmospheric oxygen is technically stable it's very reactive with most things, especially when you give it a little energy. Since life requires a ton of complex and ever changing chemical reactions to take place it really helps to have a reservoir of a very reactive substance around at all times. This also means that most atmospheric oxygen is one half of a reaction to free up chemical potential energy. So having an atmosphere made of half a battery isn't a bad thing either when talking about complex life.