Where to start and end your Simpsons rewatch is one of the most tedious discussions out there, endlessly repeated with minor variations (since you asked, I called it a day after season nine, but might not make it that far next time). Some people skip over the first year or two, if they don't have particular nostalgia for the era and their thirst for rapid-fire gags outweighs their need for stories to have a beginning, but even those who watch properly never recommend going back to where it really began with the original Simpsons shorts.
Everyone finds out about these at some point. If you weren't pop-culture sensitive or American enough to catch them on The Tracy Ullman Show from 1987–89, you may remember them from such retrospectives as 'The Simpsons 138th Episode Spectacular,' The Simpsons: A Complete Guide to Our Favorite Family (HarperCollins, 1997) or nowadays perhaps '8 Mindblowing Totally Simpsons Awesome Facts You Can't Handle Video Essay.' You marvel at the secret history, express how disturbing the character designs are (if you're a bit pathetic), then don't bother watching any more. They're not the best thing in the world.
But they matter! They may be crude, repetitive and often generic, but they're still The Simpsons. They don't always look, act or sound quite right (season one gets those criticisms too), but they're the same characters performed by the same original voice actors. The series proper gets deserved accolades, but as far as its subversion of the traditional domestic sitcom with Gen-X cynicism in a family-friendly format, Simpsons shorts did it first. And spent three seasons working out the faults so the series could explode out of the gate as a '90s phenomenon.
When you're over the shock and novelty of the frankly shite early instalments, the fascination continues as you watch three years' worth of advertising bumpers in just over an hour and the series gradually comes together before your eyes. The character designs smooth out (inconsistently), secondary characters and catchphrases are introduced (in their original context) and minimal world building is achieved. There's even a mini Christmas special that faithfully reworks an old poem. They may be demos, but it's more vintage Simpsons. Loads of it.
Before long, and increasingly, there are scenes that could slot seamlessly into the 1989–90 season and ring with a kind of alternate-universe false nostalgia for those of us who remember that era fondly. I was stuck mainly in the first two seasons for several years thanks to tardy transatlantic imports, and this is more artefacts for that time capsule of scarce two-episode tapes, irrelevant tie-in video games, speech-bubble action figures and other Bartmania paraphernalia. Its generic locations are more the setting of Bart vs. The Space Mutants than the increasingly populated Springfield to come. I even had this image on a ring binder, this is where some of that stuff came from.
The restrictive format of animated newspaper funnies meant that some details wouldn't be worked out until the substantial series, most notably Lisa being more than girl Bart, and the cartoon excess of protruding eyeballs and battling dust clouds would be toned down in the name of realism, but other differences in style and tone would take longer to come about. It's the Simpsons of the Dr. Marvin Monroe episode, not 'Marge vs. the Monorail.' For the minority who were there from the start, the transition likely seemed as natural and authentic as when the Wayne's World sketches got a movie.
More trivially, canonising the shorts means you can have more fun with the floating timeline by backdating the characters' ages by a few more years, so Bart and maybe even Lisa were originally born in the late '70s and the early flashback episodes are already retcons (Homer and Marge's cinema date should have been to the original Star Wars).
No doubt largely responsible for this material's obscurity is that the creators don't seem particularly fond of them and may be concerned about their rough quality affecting the series' otherwise impeccable reputation (pompous snort), which has blocked any official release and keeps them buried in variably lo-fi TV rips. The remastering necessary for a proper release may well not be worth it, and there may be other issues with them originating on another show, but these really belong as bonus material on a season one set. That puny volume needs bulking out anyway.
Lack of decent picture quality would be the only thing stopping me from making these formative episodes my daughter's introduction to the series in few years so she can be a Simpsons hipster, but I'll probably go for it anyway so we can watch the true development of the series unfold (until arbitrarily cutting it off when it's not warm and fuzzy any more). The Simpsons shorts count. Damn hell ass count.
Incidentally, I also got around to watching the original Wayne's World segments from Saturday Night Live recently. They were similarly fascinating and certainly stood alongside the high quality of the feature film. Not!