The walls and flooring in every bathroom that is being used on a regular basis will have some degree of moisture present. Normal bathroom walls and floors have a general moisture reading of around 3.5 to 5.5. Anything under 3.5 is great and anything above around 7.0 should be investigated and monitored.
Excessive moisture is bad for a number of reasons. It indicates that there is inadequate waterproofing or a break in the waterproofing barrier. In some cases it is a result of a well-meaning, DIY tiler who has not prepared and sealed the wet areas before doing some tiling. In some cases it is a cracked tile or bathroom hardware like taps and shower fittings are not sealed to the tiles. To ensure that a wet area is waterproof, be sure to use waterproof sealants, paints, barriers and materials.
If the source of the excess moisture is not stopped, it can lead to a number of issues, including mould, damage to paint, walls and flooring.