Illustration (as opposed to fine art) is as ancient as the cave
paintings. Fine art - to my mind - is a very personal and self -conscious artist's expression of their unique and evolving psyche. Illustration will mostly be pragmatic and generally will serve another master, be it a client or the public's whim. Prehistoric man was definitely an illustrator. He would draw and paint animals on cave walls, not because he thought them poetry in motion or wanted to "find his niche". He rendered them because it was, in his little group's collective mind, a magical way to bring all of those creatures into their campfires as food - and he was the conduit! These 32,000 year old painted animals on those dark cave walls were the first illustrated advertisements for continuing great hunts.
Thank goodness nowadays illustrators (hopefully) don't plan on killing their subject matter. Still, in a strange way, advertising spans back to those eons old hunting events. The more arresting the art, the more effective the branding. Now it's not just the small tribe that comes under the magic of a great trend - but an ocean wave of people caught up in its tide. 30,000 plus years ago, fur covered painted savages were prancing about and pointing to the cave wall paintings and salivating over the thought of a future bountiful hunt. Today, illustrators still hunt - for the opportunity to capture glory, and the big client's budget.
What is the criterion for a great illustration? It must be like a positive shockwave to the collective brain and heart; since that's what the client - across the board - is looking for: UNFORGETABLENESS! Witness Coca Cola's Santa Claus, painted back in 1931 by artist Haddon Sundblom. He created an American icon forever linked to Coke. That snowy bearded man in a fire engine red suit, with a button nose and irresistibly cheery expression, became the best beloved template for every ensuing Santa illustration down the decades.
Then there's Norman Rockwell's "Thanksgiving" painting - a classic branded into every American's brain and held as the ideal rendering of a family holiday gathering. Siegel and Shuster's Superman, that caped muscle-man who could move mountains, and catch a falling passenger plane in midair, started a slew of iconic knock-off superheroes.
Although most really good art involves fulfilling the client's vision to the letter, every red-blooded American illustrator strives in their heart of hearts to come up with that never to be forgotten masterpiece, often emulated by their envious competition - beloved and cherished by millions.