I can think of a variety of ways in which rank are useful to everyone but the rank holder.
Rank tells me, as a relative newcomer, who I should ask in our dojo about specifics of technique.
Ranks gives me a rough approximation of the intensity with which I can execute, or expect a technique to have.
Rank lets me know who I should speak to about matters of the up-keep of the dojo, practice schedules, etc.
This all assumes that rank is not handed out willy-nilly, that that those who hold various ranks understand the responsibilities they have, and act to fulfill them.
Outside of our dojo, the context is lacking, and it becomes much less meaningful. We often have dan-ranked guests who have poor technique, and some who have excellent technique.
I can also think of one way in which rank is useful to the rank-holder--Sensei is telling me that I have new responsibilities, and he trusts me to execute them properly.
I have never seen anyone in our dojo use their rank as a tool for ego gratification, and I would of course be aghast if that were so...