What is the purpose of adding polyethylene film in interior slabs sitting on grade for building structures?

Membrane materials, such as polyethylene film, are commonly used to reduce vapour transmission from soils to concrete slab. They are often termed “vapour retarder” and are placed on the underside of concrete slabs sitting directly on soils for building structures. Protection from moisture is essential because floors inside buildings are normally covered with carpet and tiles and penetration of water vapour through concrete slabs could result in the failure of adhesives in tiling, discoloration of flooring products and fungal growth.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.