(or "CARR! What Is He Good For? Good God, Y'All!")
The storytelling engine for 'Justice League of America' is another one of those "inevitable ideas" that every comics company comes up with sooner or later; we have N comics that sell X copies each, therefore a comic that features every single one of those characters in a team will sell N times X copies. So the heroes of the (fill in the blank) Universe team up to more effectively promote truth, justice, and heroism in between their exciting solo adventures.
In this particular case, the Justice League is a newer, slicker version of the Justice Society from the 1930s and 40s. (Editor Julius Schwartz famously thought that "Society" sounded snobbish, whereas "League" brought to mind America's national pastime of baseball.) As the JSA featured the popular characters of that era, so does the JLA feature the Silver Age's most popular super-heroes--Batman, Superman, Wonder Woman, Green Lantern, Flash, Aquaman, the Martian Manhunter and Snapper Carr.
"Wait, what, who?" I hear the casual comic fan asking. (While the longtime comics fan says, "Oh, God, not Snapper Carr...") Snapper Carr was an attempt to include an audience identification figure; after all, a book like the Justice League, with a large ensemble cast of super-heroes has less room for a supporting cast than a solo book. So on their first adventure, a teenage beatnik named Lucas Carr, nickamed "Snapper" for his habit of snapping his fingers to indicate he likes something, helps the League defeat Starro the Conqueror with his high-school level knowledge of starfish biology. As a sign of their appreciation, the Justice League makes him an honorary member.
To many (perhaps most) comics fans, Snapper is an annoyance, something of a throwback to an earlier era of comics where every hero needed a "kid sidekick" to give the kid readers someone to identify with. (See also "Jones, Rick.") Every time Snapper pops up, answering the JLA's mail or listening to their stories or getting shanghaied on one of their adventures, people who take comics seriously as an adult medium feel like they're biting down on tinfoil. (It doesn't help that his dialogue is "beatnik" jargon written by people who've never actually talked to a beatnik: His first line of dialogue in the series is "Man, this grass mat is the coolest! Wait'll Daddy-o casts his orbs on it!")
But Snapper does perform a valuable service to the Justice League's storytelling engine. Not as an audience identification figure, though. I can't imagine anyone reading about Snapper Carr and saying, "He's just like me!" No, Snapper Carr is useful as The Guy Who Doesn't Know What's Going On. This is a vital character for any truly long-term storytelling engine, and variants of it pop up all over long-running series. Why? Because storytelling engines are all about things that recur, about elements that get reused because they've got potential for more than one story in them. But every story is somebody's first. Having a character who doesn't know the backstory gives the writer a chance to deliver important exposition without it seeming forced or awkward.
There's a long and noble history of Characters Who Don't Know What's Going On. Kitty Pryde in the X-Men (or Wolverine in the X-Men movies), Justice in the Avengers, every single Doctor Who companion, the list could probably go on forever. These people aren't necessarily "audience identification figures" in the sense of being unusually sympathetic or "normal", but they are people who act as surrogates for the reader who's coming to this universe for the first time and don't necessarily know who Doctor Light or Chronos the Time-Thief are, and why they might have a grudge against the Justice League. Without them, the comic becomes insular, catering only to a relatively small audience of fans who already know the intimate details of the fictional universe. Good expository writing helps open up a series to new fans, and characters like Snapper Carr, who don't know anything about Earth-2 and need a quick and convenient explanation, help make for good expository writing.
Mind you, they become more important as we move into the Bronze Age. Back in the Silver Age, writers thought nothing of simply dropping large chunks of exposition right into the story, either in the form of narrative captions, or random info-dumps spoken by the characters. "Yes, Robin, it's our old enemy the Riddler! His gimmick of coming up with clever riddles to taunt us with information about his future crimes has nearly been our downfall, and he's escaped from the jail we put him in after our last encounter!" That sort of thing.
But by the time Snapper was really needed, he was gone. Just as unmotivated infodumps were a casualty of the end of the Silver Age, so too were kid sidekicks and audience identification figures. Snapper betrayed the JLA to the Joker, was forced to resign his honorary membership in disgrace, and has been reduced to an occasional cameo appearance in random comics. Can it really be a coincidence that comic book readership has declined ever since?
Well, yes it can. Snapper himself isn't important, only what he represents. Comics need a person who isn't as familiar as the writer with the details of its universe, or otherwise they'll only appeal to people who already know everything about the comic. And in the end, that audience is doomed to dwindle away.