With gravity, water will always find the easiest and quickest route downwards. If a weak point or crack develops in your floor or walls, or gaps form in the sealant between your walls and bath, water can easily start to seep through and will soon find its way into the ground floor of your house. This may manifest itself in damp walls and ceilings at a lower level, or worse, drops of water forming on light fixtures, meaning water and electrics mixing, which is never a good thing. A tanked room provides a fully watertight seal beneath floors and walls that will prevent this. Not only will waterproof boarding prevent leaks and damp forming, but it is also a great insulator. If external walls form part of your bathroom, you will know how cold those walls can feel to the touch, especially during the winter months.