The Desert as an Economy
Take the desert out of the story for a moment and the plot becomes much easier to see. What survives is a supply chain. One substance, produced in one place, needed by everyone, worth more than the place that makes it. Every other event in the first novel is downstream of that arrangement, including the death of a duke and the marriage of his son.
The spice is the only thing the whole civilisation genuinely depends on. It is not a luxury and it is not a metaphor; it is the unit in which wealth is counted, the reason interstellar ships move, the thing the Emperor's economy is denominated in. A house rises and falls with its stock and its contracts. The man who controls the flow of the substance controls the price of everything else, and so the desert is not scenery. It is the only place in the universe where a single point of failure sits.
The control is indirect, and that is the interesting part. The Guild does not farm the substance; it moves people and goods across the gap between worlds, and it takes a percentage of everything it carries. The families keep the Houses, the Council decides how the shares of the corporation are voted, and the Emperor sits above both with a lever in the form of a withheld vote. Nobody owns the desert outright. The arrangement is a set of balances, and the first novel is largely about what happens when one house gains enough weight to tip them.
And then there is the water, which is where the desert turns local. On Arrakis every drop has a price and a politics attached to it. The condensers are private, the harvest is guarded, the sellers know exactly how much thirst their customers can afford. The planet sells its scarcity to the people who live on it while the outside economy sells the scarcity's product to everyone else.
This is why the desert is a market before it is a wasteland. A wasteland is a place where nothing happens. This one has landlords, taxes, contracts, engineers, and a harvest that keeps going whether anyone approves of it or not.