I understand you can say either
I'm president of the United States.
I'm the president of the United States.
I also know of a rule that you may omit "the" in front of a title, position, etc.
Since the omission does not seem to depend on the style of the text, I'd like to know what is the reasoning behind such a usage and rule.
Is it because you tend to consider a title something like a proper noun?