Really, from my understanding, you're right: it should not make sense. It's pretty much an absurdist book, it was adapted from a radio show and was more or less just comedy sketches centered in a ragtag, nonsense setting. None of them really have much to do with each other, nor do they really build off of each other, but I find Douglas Adams writing and ability to come up with what he does to be endlessly engaging and uplifting.

You mentioned judging the scenes by themselves; this is how I view Hitchhiker's Guide, as a compilation of great scenes that each occasionally do have some great meta-commentary on the world at large, and almost all are funny and self-contained. There isn't so much "development" as there is a progression of in-jokes.

The characters are more or less used as puzzle pieces to fit into a scene; while they might not really "develop" so to speak, they are fun archetypes that are thrown into silly situations, and we recognize them by what they do and say. They don't compare to most novels or literature when it comes to character development, but it can often be a gag as well just how far they've come without developing whatsoever.

It definitely does try hard, and it definitely is incoherent. This turns it off to a lot of people, and that's totally reasonable. For those who essentially want to read Spaceballs, however, it is a work of art comparable to Plato's Republic.

It's comedy. Most sci-fi and fantasy novels are serious, so having a comedic bent is unusual, which is interesting.

Beyond that, though, comedy itself can be interesting because it looks at life from a different perspective. Much of comedy is based around the idea of looking at the mundane aspects of life, and getting a fresh perspective.

Why do we drive on 'parkways' but park on 'driveways'? etc.

We have governments, and bureaucracy in our daily lives.

What would that look like if you extended that concept to an intergalatic civilization? You'd have a race of bureaucrats that end up destroying our planet to make room for a new construction project.

It's common for sci-fi stories to focus on the fantastical and awe-inspiring aspects of the story, like teleportation and faster-than-light travel.

This, however, focuses on the mundane aspects of life. Yes, there are fantastical elements to the story, like 'improbability drives' and such. But the focus of the story is on the banalities of life. The daily quirks and joys and annoyances that affect us all.

Arthur Dent is dealing with local red tape, and the local pencil-pushers try to destroy his house to make way for a highway. His universe then grows dramatically as he learns that there is a vast universe out there, teaming with intelligent life. And guess what? He's still stuck dealing with bureaucrats that want to destroy his home.

Life isn't always about psychic powers and laser guns. Sometimes it's just about crushing on a girl that you like, and having her run off the with asshole with a better ride.

That's an interesting story, even if the asshole has two heads and the 'ride' is a spaceship.

Yeah ... HHGTTG is like a super extended standup routine or vaudeville act. Reading for "coherence" or "an idea" is missing the point.

Books that set out to be comedy don't really work for me. HHGTTG is probably the best I've read but even that was just... tolerable. Biggest laughs I've had came from "normal" fiction, sprinkled in here and there. Dickens is good at that. Best laugh I've had from a book may have been from a set up chapter followed by a punchline first sentence of the next in Lynch's The Lies of Locke Lamora, a book that's otherwise occasionally funny but not at all a comedy. I had to put it down for a while to stop laughing.

By contrast, comedy's almost the only thing that works for me in silent film. And not just the slapstick (though that's great) but also visual-narrative comedy. That and mind-blowingly-large-scale spectacle, like the epics usually manage for some part of their runtime. Serious, personal drama? Nope. Books or talkies for that.