First time poster, let me know if this is not about world building.
I am not asking about the function of respiration nor the chemical effect of oxygen and stuff. I am asking if there is any good reasons to force my creatures "to repeatedly perform an action for their whole lives in order to maintain their lives, while such action does not require any extra input and is essentially free and involuntary".
While these continuous life supporting actions include also eating, drinking, resting and excreting, all these require extra work or effort (e.g. hunting, sheltering, cleaning), only breathing is basically free. Air is provided, no harvesting is needed. Supply is virtually infinite, no over consumption issue, hence no war, no fighting because of resource depletion.
So why would I create creatures in my world requiring them to perform this seemingly unnecessary action to consume an infinite resource provided for free?
It seems to me this is just a mean to separate the territories of my creatures, e.g. terrestrial vs aquatic creatures. While the resource is infinite, the tools capable to consume the resource can be different, some specialised for one, some both. Another reason I could think of is to limit the movement boundaries of my creatures by setting up a space that does not have this resource, so that my creatures would hold captive their entire lives within an enclosed place.
Those are just my thoughts though. But then again, as civilisation advances, my creatures would overcome this limitation and start colonising in other territories or even planets, so it seems this is only there to delay this process? I hope there can be some important reasons behind to justify doing this that I overlooked?