We don't have to. Though I suppose it can make life easier at times. I read once that some financial guru guy doesn't wear watches and just lives by his own time (he eats when he feels hungry, sleeps when he's tired, works on a project till he feels finished for then, etc) and I thought that was interesting because other than having to get my kids to school on time, I usually don't think about time either. Or maybe it was that I didn't used to. I do more these days though.
Oops, Do you ever wonder just how much of life we don't yet understand?