WinCustomize is the only site currently where any ranks beyond citizen, and moderator matter. On WinCustomize certain site functions are restricted to higher-level users (such as rating skins, submitting work without moderator review etc.) But on sites like JU, ImpulseDriven, Sins, Galciv etc, the ranks don't have any technical purpose.
On GalCiv 2, back when the game was originally in testing, there was a forum only accessible to Diplomat users or higher (rank 3), these were our gamma testers who helped give more directed feedback at the end of the development cycle. We now have a separate system for private testing that supplanted this.