Moisture vapor and soil gases are driven into the building envelope by diffusion. If you don't take the ground out of play with an effective below-slab vapor barrier, then moisture, gases, and contaminants will move into your building envelope and negatively impact your indoor air quality, energy efficiency, and building flexibility.
The best rule of thumb is if you have a conditioned building, then below-slab vapor protection should be part of the building design regardless of flooring materials.