The spice prolongs life, in a way they are unable to replicate or otherwise export production of. Production of the Spice also is devastating to the ecology of Arrakis (nee Dune). The natives haven't tamed the worms - but do co-exist with them. The Houses exploit both however, and the planet itself.
The guild navigators value is NOT they can perform computations machines cannot - it's NOT a complexity problem. Some spoilers so I will be oblique, but their ability is tied directly to what the Bene Gesserit are trying to breed. And to what Paul is/his abilities are. It's inherently an "organic"/meat-puppet power - as it is implied it's tied to forebears and progeny...
And finally, as noted, there is a deeply held proscription against "thinking machines" due to an... event. This actually isn't HUGELY relevant, but allows Herbert to eschew futorology in favor of his allegory.
The story's GREAT :) (especially up to and including Book 4, which completes a full arc.)
Also the whole setup is obviously allegorical. There was an oil shortage in the 70s, so the setup was a play on that.
IIRC there is a reason for that: most computer systems were destroyed and their use, banned.
It was interesting and very different to imagine a far future where there is great technology but no "thinking machines" due to a previous human machine war (Butlerian Jihad). You see this concept reused in later sci-fi (Terminator, The Matrix). Then exploring how you could replace computers with the human mind given some help (spice).
Control of the galaxy is determined by intergalactic space travel, and to do that, you need “The Spice”, which is the shit from these giant sand worms, that are tamed by the locals of that planet.
Then, to add further suspension of disbelief, you have a guild of these space folding specialists that uses the power of their brains to conjure up the space warp travel, because apparently a computer system couldn’t handle the complexity of the physics of space travel. The whole premise falls apart, and dwells in nonsense.
I can’t imagine what is so good about this story, except that it is some space soap opera from a lost generation trying to make its mark.