It's how they deal with not having the budget to actually put people in a desert so it's rather visibly a desert backdrop at some points, it's why the lighting is often so strong and monochromatic like a theater's lighting, it's why the acting is with all the characters projecting as if they are speaking to an audience rather than in a TV production. At one point they even use a very theatrical special effect based on backlighting through a thin cloth representing a tent to show Paul's hallucinations/thoughts, rather than even a simple bluescreen or conventional image compositing effect.
The effort is interesting, but I'd have to call it ultimately a failure (even if I personally ultimately enjoyed it) as it almost completely failed to speak to its audience. I also think you can't really follow it if you didn't read the books, which I consider a major flaw of an adaptation.