This is especially true in the case of superhero vanity. It seems like the moment a superhero decides they will use their unique abilities and resources for the good of mankind, they then select a name and a symbol for themselves. And shortly afterwards, the superhero goes around plastering this name and symbol over everything they own. In some cases, it seems like the superheroes concerned are building a brand and increasing their own marketability by the manner in which they promote their heroic image.
Professor Xavier has "x"s all over his home and his students. Granted, these "x"s also refer to the "x-factor" in the genes of mutants, but I'm sure Xavier gets a certain degree of pleasure when he realizes that his last initial is emblazoned across Emma Frost's chest. Not to mention belts, guns, costumes, communicators, and an entire series of supersonic jets. The Fantastic Four use their special digit to stamp their flying car and (sometimes) even the building they live in.
The worst offender by far is Batman. Bruce Wayne created symbol of a dark knight defender whose mere presence would strike fear into the hearts of criminals. And then he integrated that symbol into every element of his crime-fighting arsenal. The bat-suit with its flowing cape and chest insignia letting you know just who you're dealing with. There's the batarang which can be thrown from long distances and left behind at a crime scene. The batmobile can be seen streaking through the night. Everything he uses is shaped like a bat. He even gave Commissioner Gordon a spot light marked with his symbol to shine in the night sky. The implicit purpose of all of this is to build the Batman mythos and give criminals the impression that he is everywhere. And though I still believe it to be incredibly arrogant to make everything you use shaped like bats, it's possible that doing so will deter criminals from engaging in illegal behavior by leaving behind these calling cards to show the consequences of committing crimes in Gotham City.
But then there's the Bat-plane.
Who even sees this vain aerodynamic monstrosity? Have you tried looking for a plane going Mach 3 while looking in between tall buildings? It's certainly not the most salient object in the night sky. And who exactly is deterred by a bat shaped airplane at 20,000 feet? Villainous 747 pilots and ill-tempered pigeons? No, the Bat-Plane is a sheer exercise in vanity. Any other shaped aircraft work just as well (or better) and would not require the needless expense of matching the design of a 6-million dollar vehicle to the little symbol you drew on your bat-chest. True, as a rich playboy, he's already the picture of excess, but why not buy a more maneuverable Apache helicopter from army surplus and use the leftover money to install more motion detectors and reinforced doors in Arkham Asylum?
So, regarding the Bat-plane, its utility is questionable but its ability to satisfy Bruce Wayne's personal bat fetish is undeniable.