Any movie production has extras. The same should apply to comics as well. Wait....It does!
Extras have a very basic use in comics: They not only act as window dressing, but they fill things out. Extras are those characters that do nothing, yet are all around. Those characters in the background haggling or just walking around? Those are extras. The guys ordering beer that we'll never see again? Extras. The geeks being slapped in the foreground that we have no reason to care? Extras!
Extras are characters that can be used for humorous purposes, or to establish that world is populated by more than the main characters. If you want to slip a joke in, or a subtle bit of exposition, your extras come in really handy. They can also be useful to establish that your world is busy as the real world is, or to show how extravagant a party is. Admittedly, they aren't a writer's issue, but the writer should bring them up to artist.
Also, bear in mind that there several advanced types of extras. The first type is the “mook”. You know those characters that all the heroes wade through on the way to the main action? Those are mooks. Mooks are the ultimate window dressing for the bad guys; they do all of the common tasks that the main bad guys have neither the time nor inclination to do, such as balancing checkbooks or kidnapping sidekicks. The palace guards, or any other character that is just window dressing and usually appear en masse are also mooks.
There's also insignificant characters, which are just modified extras. They may have a use, but they are usually just background characters. “Nurse Joy” from the Pokemon series, for example: She may be a running joke (every town has one!), but she has no real effect on the plot, and her capabilities (outside of medical and causing Brock's heart to race) are not well-developed. As such, she's basically still just an extra. The local barkeep is another example, as his staff. They may give the tavern a certain stability, but they are only useful as far as the tavern is concerned; they are useless (plot-wise) outside of it.
Oh, and let's not forget “fonts” (short for “font of information”). These are characters, like snitches, sages, and the occasional barkeep, whose sole reason for being is to dole out information and get quests started. Cliché, but very useful.
By applying these characters liberally, you can create a world that is more detailed than one that just holds the characters. It's neat to have powerful well-developed characters, but you need more than that in order to really have fun with the comic!