I think that a problem artists have, when trying to deal with the world, is that we tend to know ourselves better than we know anyone else and we tend to be a cerebral, analytical and, to borrow an old term from Woody Allen, "verbal," lot.  David Foster Wallace was, artistry aside, a genius level intelligence.  He was not only extremely facile with the English language and possessor of a large vocabulary, but was highly educated in arcane modern branches of philosophy and mathematics as well.  He also had excellent understandings of music and competitive sports.  He had an extremely broad mind but one so capable that he did not have to necessarily make the sacrifices of the dilettante in terms of his depth of understanding of various topics.

For a writer seeking to create realistic characters, this can create problems.  If the author wants to describe the world a certain way but the character could not possibly, then the author either has to sublimate the writer's voice (often a very good decision, but not a painless one) or risk presenting the character as something less than authentic (often a very bad decision but it can work).

This is why a lot of fiction and a lot of movies tend to be about "smart" people and child prodigies.  Think 90% of J.D. Salinger here or movies like The Royal Tennenbaums. If the characters are all super geniuses then they can say whatever the author wants to say.  Heck, we the reader imagine that the characters are somehow smarter than the author and this gives the author free reign.

But worlds full of super geniuses are basically a type of science fiction.

By using footnotes, Wallace seems to have found a work-around.  We see this at work in the famous "Eschaton" scene at the Enfield Tennis Academy.  Eschaton is a global annihilation nation-states type of game that takes place on four tennis courts and utilizes blasted out tennis balls as nuclear weapons.  A scenario is constructed by the game master and the various participants go at it, attempting to destroy or take each other's territories.  But, these are children, after all (though very smart, see above) and so in this instance, the game erupts into a massive fight.  Rather than targeting territory, one participant (Ingersoll) takes aim at another player and as the physical fight gets going the philosophical fight over what the rules of Eschaton are and must be also heats up. The game master is Otis P. Lord and the philosophical conscience of the game is Michael Pemulis:

"Pemulis howls that Lord is in his vacillation appeasing Ingersoll in Ingersoll's effort to fatally fuck with the very bread and breath of Eschaton.*

*Pemulis doesn't actually literally say "breath and bread."

I think this is a pretty awesome compromise.  The author gets his say but the character gets a correction in the footnote. Later on, in a passage describing a Boston AA meeting, we get a footnote from the point of view of Don Gately: "None of these are Don Gately's terms." because Gately is definitely not the type who would use a phrase like "unspoken, Shamanistic Fiat."  However, just prior to that, Wallace has a little fun with us because another character, from Gately's point of view is described as looking like Truman Capote and we know that Gately would not likely know what Truman Capote looks like except in this case the footnote tells us that Gately does know and why.

The footnotes are more than just asides.  In the Eschaton section, which is described in a limited way through the points of view of both Hal Incandenza and Pemulis, the Pemulis character actually gets full voice in the footnotes. The most substantial of them, which describes the algorithms used by the Eschaton game master to cook up the scenario and measure the damage, failures and successes of the attacks is completely written by Pemulis.  So this device isn't just a clever way for the author have asides with the reader. But it manages to allow both readers and characters to say what they want to say and so becomes a valuable artistic device.