A somewhat smaller supposition that I know nothing of price action. And yes all derivations of price computated constitute the basis of an oscilating indicator.
A true expert will know why. The various tools will be used when and where required. Watching bar by bar only, has its limitations.
Your final sentence highlights the ignorance of and disregard to various strategies and tactics that price action (bar by bar watching) alone will not assist you with at times.
I am not an expert, let alone a true expert, so perhaps I'm not able to think of the reasons myself. I see indicators as no more than a visual aid for representing what can already be found on a chart. Since you know what a true expert would know, would you care to step off your high horse and at least provide some reasoning?