What I want to talk about here is that they are not some inherently new gimmick, devised by Hollywood executives who ran out of Sequel ideas. They are just as much an established part of Literary tradition as Sequels are.
I could go on a long diversion about the ambiguities of applying modern terms like Sequel, Prequel and Reboot to ancient, medieval and renaissance literature, poetry and theater. But I don't think that's necessary.
The Sequel in the sense of how we mean that in talking about modern Nerdy Genre Fiction was probably invented by Alexandre Dumas, as he and his French peers laid all the foundations of modern Popular Fiction. He wrote sequels to many of his works but by no means all (he never did a follow up to The Count of Monte-Cristo). But the ones that probably mark the birth of the modern Popular Fiction Sequel would have to be his sequels to The Three Musketeers. In fact you could even call the D'Artagnan Romances the first modern Trilogy.
The Three Musketeers novels are among other things literary ancestors of Star Wars. The Buck Rogers and Flash Gordon style serials that inspired George Lucas and the Pulp fiction they were based on were largely predicated on taking the tropes of the Swashbuckler genre codified by The Three Musketeers and placing them IN SPACE. Star Wars Rebels I feel was very much acknowledging this.
The Three Musketeers sequels are at least to the English world not quite as popular and well known as the original. Thing is Dumas also wrote Prequels, and it was his Prequels that had a tendency to surpass their original.
Le Chevalier de Maison-Rouge was the first written but chronologically last of what are called the Marie Antoinette Romances. It was successful and influential enough on it's own to be a direct inspiration for The Scarlet Pimpernel. But the first two books, Joseph Balsamo aka Memoirs of a Physician and The Queen's Necklace were a far bigger deal, as relatively unknown in English as everything of Dumas but his big two, but in the French World they have come to define the legacy of Count Cagliostro. And like the Star Wars Prequels they were the story of a Villain more so then of a Hero, they played a vital role in creating the modern Super Villain, and I see much of Joseph Balsamo in Darth Sidious, (an argument can be made that Althotas is the prototype of Darth Plagueis).
Also The Whites and the Blacks is a prequel to The Companions of Jehu.
Next in line is Eugene Sue. His Les Mysteries du People series was his intended magnum opus. The volume set during the original French Revolution, The Sword of Honor, became also a prequel to both Les Mysteries de Paris and The Wandering Jew, setting up the hero of the former and villain of the latter. Then the next and last book of the series set during the 1848 Revolution, became a sequel to both said novels. So his main Prequel came before his main Sequel.
Paul Feval's first significant success was Le Loup Blanc (The White Wolf), which is arguably the first masked vigilante. Later Feval wrote a prequel to that called La Louve (The She-Wolf), which is pretty indisputably the first female masked vigilante. So that is a pretty important literary innovation that was done in a prequel, he retroactively made his Heroine predate his first Hero.
Later Feval wrote Salem Street and The Invisible Weapon, to be Prequels to the main narratives of the original three novels of The Blackcoats saga. However the continuity divergences make them more like an alternate continuity, or a different timeline created when Barry Allen went back in time to save his mother creating a Time Boom (and the disappointing Cadet Gang can be the New52 canon). The Invisible Weapon is generally considered the best Blackcoats novel, by modern fans at least.
Emile Gaboriau created in his 1866 novel L’Affaire Lerouge the character of Monsieur Lecoq, in this first novel he was a supporting character but he became the lead in later stories, he was a French Master detective who predated Sherlock Holmes. In 1869 he wrote a prequel novel titled Monsieur Lecoq about his first major case. This prequel quickly became the most successful of his novels, so much so that many think the reference to Lecoq in A Study in Scarlet was based on this novel alone.
The Holmes stories were not written in chronological order ultimately. So much so that all four full novels are set before The Final Problem though half were written later. None are a prequel in the traditional sense since none give Holmes an Origin Story or set up any story-lines we'd already seen the ending of. Still Hound of the Baskerviles tends to be labeled the best Holmes story, yet the reader knew even when it first came out Watson will live to witness The Final Problem, so he was never truly in danger.
The son of Paul Feval wrote a series of interquels to the D'Artangan saga, set between the first two novels. Known as the In Between Years series.
However with Arsene Lupin we again have a prequel origin story novel, La Comtesse de Cagliostro, that becomes possibly the most popular of all his adventures, it's the main basis of the 2004 movie and a superficial influence on the Lupin III Anime film Castle of Cagliostro. And fittingly this novel's lore is largely derived from Alexandre Dumas' most important Prequels. I highly recommend the English translation done by Jean-Marc & Randy Lofficer for Black Coat Press titled Arsene Lupin Vs Countess Cagliostro.
Other Lupin books written after WWI were also set before some of his earliest adventures, like The Damsel with the Green Eyes. But in fact the first Lupin Prequel was The Crystal Stopper, written after 813 but set before his major Holmes crossovers.
Now applying the term to Tolkien's Legenderium can be controversial because the first age stories were what he began writing just never published. Regardless creating Numenor, then The Hobbit and then LOTR caused him to change things about the first age. Sauron was really created for those later stories, in the oldest version of Berne and Luthien (the only First Age story Sauron is important to) his role was played a sort of WereCat.
Regardless the basic argument against prequels still applies, when they were finally published the readers knew Morgoth would be defeated in the end and that Galadriel wouldn't die, and that Numenor would end up being Atlantis. And so on.
All of these are parts of sagas and from writers that laid all the foundations of modern Popular Fiction, in more then one way I could argue we'd have no Indiana Jones without Arsene Lupin. So Prequels were a part of the kind of fiction Lucas was making long before he made the decision to call his 1977 film Episode IV.
So let me clarify my premise. Without certain Prequels that were popular and influential before George Lucas or his critics were even born, we would not have Star Wars or Indiana Jones at all.
Some sagas shouldn't have prequels because arguably doing a Prequel is a symbolic betrayal of what it's about, like Star Trek, I never supported Enterprise, or the 2009 film even before I heard it was an alternate timeline Reboot rather then a Prequel, nor the new series being set before TNG. I want a 25th Century Enterprise with a Pansexual Korean Woman as the Captain.
But Star Trek and Star Wars are in many ways opposites (one is SciFi and the other is Fantasy) including on this very issue.
Star Trek begins with
"To boldly go where no Man has gone before".Star Wars begins with
"Long Long Ago".Star Wars is about the past, it always has been. Therefore I never really cared about post ROTJ, hell I barely care about post ROTS. The only spin-off idea thrown around I'm interested in is the Young Yoda film, because that can take us centuries into the past, to what was "Long Long Ago" to the people of Episode 1. And Maz Kanata is older then Yoda, so you can throw her in there to appeal to the New Trilogy fans.