"What? Harmless? Is that all it's got to say? Harmless! One word!"
Ford shrugged. "Well, there are a hundred billion stars in the Galaxy, and only a limited amount of space in the book's microprocessors," he said, "and no one knew much about the Earth of course."
"Well for God's sake I hope you managed to rectify that a bit."
"Oh yes, well I managed to transmit a new entry off to the editor. He had to trim it a bit, but it's still an improvement."
"And what does it say now?" asked Arthur.
"Mostly harmless," admitted Ford with a slightly embarrassed cough.
-- from The Hitch Hiker's Guide to the Galaxy by Douglas Adams
The book Mostly Harmless is many things. Most obviously it's an attempt to tie up all the stray plot points from the "increasingly inaccurately named Hitchhiker's trilogy" and a chance to see Arthur and Ford back in print once again. It's a loud knock of absurdist existentialism for those Douglas Adams fans who didn't pick up on that aspect of the first four "Hitchhiker's" books. It's a new Dirk Gently novel in disguise, at least if you go by tone and plot style. It's funny, it's sad, it's depressing, it's confusing, it's incredible, it's unique. It's essential Douglas Adams, in other words.
I like to think that in a metaphorical way, the Vogon commander in this novel is Adams himself. They both keep trying to destroy the Earth (in the books, anyway) and end the stories of the characters' lives. They both keep watching it pop back up, in some other kind of reality, despite their best efforts. They both need to have a bunch of loose ends tied up in order to do so. And so they get to work.
No one's really happy with how it all works out in the end, of course. But you have to have a good laugh, as Ford does, when you understand what it's all been for. It's actually quite brilliant how the commander/author conspires to make it all happen. Nobody could have seen it coming. It's the end of the world as we know it. All for an interstellar bypass, or a bit of bookkeeping, or maybe just for the heck of it.
Q: What's the Answer to the Question of Life, the Universe, and Everything?
A: Forty-two.
Q: What was God's Last Message to His Creation?
A: We apologize for any inconvenience.
Q: Who will be responsible for the end of humanity?
A: A military bureaucrat with a checkbox to fill.
It doesn't always have to be about conquering the galaxy, after all. In reality, it's usually because it's what someone told you to do. Too anticlimactic? Get used to it.