I know what I like about Sunnydale as a small town cut off from the rest of the world...not only does it make it easier for me to suspend disbelief, but I have a fascination for "twilight realms" that are connected to the world, but not fully part of it.
It's not uncommon in many stories that there is such a place that serves as both a menace and protection from the rest of the world (and I loved how in The Chilling Adventures of Sabrina that you could tell those new or passing through as they had iphones while the locals seemed to be more in the 80s techwise, and I loved the more extreme concept of Storybrooke in Once Upon a Time).
It attracts and holds demonic creatures (without actually imprisoning them, usually). That is to invade the world, they first have to conquer this particular piece of real estate, one in which a town can form without knowing the truth of the area (and social and psychological forces would impede any full awareness or talking about it, and I imagined most of Sunnydale PD being transferred there as a punishment who'd then become much like how they're portrayed).
Magic works easier here (witches and supernatural critters probably feel the difference when they enter, invigorated by it, but knowing that they have rivals also invigorated by it). The laws of physics are just a bit wonky (from the mild seepage of physics of other dimensions, and when made stronger then it becomes more pronounced, like rain of toads, blood from faucets, etc.).
That same trait allows "super science" (androids, ray guns, mutagens, etc) to work, but not to transform the world because they inevitably suffer fatal flaws when taken out of the area, while magic itself is weaker (not only would those like Marcie become visible once under government scrutiny, but they probably couldn't turn invisible again until in a magic-rich area, and the swim team turned into fishmen would need to find a "Bermuda Triangle" spot or likely find themselves dying as the mutagen turns cancerous, as it did before Sunnydale, thus explaining why such didn't seem to exist outside of it in a stable condition).
A demon biker gang can feel they can let loose in Sunnydale (raping and pillaging in the burning town) because the National Guard won't be called (at least not right away, and then they send in the special units).
'Course people like the Mayor and even Master would frown on unsanctioned violence on their turf as challenges to their authority and mastery (and I liked one book where the Mayor tolerated Slayers because they got rid of some competition for him, and only later did he want them removed from the equation).
But all in all, I like the canon to remain consistent (it can change, but there needs to an in-story explanation for it, though magic can always work), I can enjoy the super science without wondering why it doesn't change the world (as it would break down once taken outside since there's no seepage from other universes that allow it to work as well), don't have to worry so much about why feds weren't all over Sunnydale High over the destruction of the school (and given all the kids involved, I think it would be impossible to hide Buffy and Giles, both who would be under immediate suspicion anyway, along with some spotty police records in regards to Buffy), and..most of all...I like to think of it as "a twilight zone" that is slightly "out of phase" (but still reachable, at least safely by car or foot) from the rest of the world as it makes it feel more cut off, and thus more dire (especially as there's very little help from outside, and if Sunnydale falls, then the invasion of the rest of Earth begins from there, mushrooming out).
buffy.fandom.com
. Sunnydale is like Stars Hollow in Gilmore Girls or Cicely ,Alaska in Northern Exposure , a dreamlike setting that expands and contracts to suit the story . I like to fanwank that it was all arty and deliberate : rather than just careless writing 
