If extras are those characters that are just there to provide filler, you will have those that perform specific tasks that show up all the time, but aren't part of the main cast. The perfect example of this is Nurse Joy and her infinite sisters from the Pokemon universe; every town has one, and her function is to merely heal pokemon and show how big of a pervert Brock is.
The “font” extra is just an extra; his or her identity changes each time, and the actual person doesn't matter. That is, it's a generic seer, a thug found in a bar, or just a random someone you pick every time you need a specific type of information dispersed. In this situation, the character doesn't matter, and so he's still just an extra.
However, you need to show that your characters have real effects on the world. By having a real person having been affected by the characters' actions, you show that your world has depth. And in order to do this you need recurring extras. If your characters eliminate hunger, you need to show that someone who was famished is enjoying a good meal; it really brings home the point that they have done something (eliminated hunger) and that it has had an effect (people are eating).
In cartoons, recurring extras are used extensively. You have the poor boy who the hero treats with respect who ends up saving the hero's life (usually as a sacrifice). There is the mother of six that has adopted the heroine, usually for succor. And don't forget the caravan owner that seems to be everywhere...
A recurring extra should never become an actual permanent character; occasionally you can highlight them, using them as a plot hook in order to get a character into the story, but the character should then disappear into the woodwork. They should only show up when you need someone from the world to show that your characters are having an effect; bring them up early in the story, and then when the effect has occurred. You can show the character as events change things, but don't get too crazy with showing the character; he may be a human being, but he's still just part of the background.
Keep in mind that he's there just to create some kind of sympathy and to show how important the character is; he's not there to add to your cast, but how your cast is perceived. Sometimes it's hard to create sympathy for your characters; they may be extreme anti-heroes, cold, or extremely professional and are thus unable to truly interact with other characters, and so showing the human side of your characters is difficult. By creating an extra that shows that the characters are human, and that the character is cold for a reason, you create a little more sympathy for your characters. Without that human touch, your comic just doesn't have any relation to your audience. And without that touch, your comic just won't become popular. And you want to become popular, right?
So remember those single mothers with large families, roaming caravan owners, and sacrificial urchins; you may need them!