The largest part of a luxury timepiece is the case, which connects to the bracelet. Together, these two components make up the outside of a watch. The main job of the case is to protect the inside movement, as well as the dial and hands.
These parts are all easily damaged by everyday activities and the elements - especially water. Even if you are a professional diver or someone who goes yachting on the weekends, you come into contact with a lot more water than you would expect: washing your hands, giving the dog or the baby a bath, getting caught in the rain, doing maintenance around your house and putting your hand in a bucket of water - not to mention your normal sweat and humidity in the air.
To combat the deluge, luxury watch cases always include gaskets. These little rubber rings create a pressure seal that keeps the movement free from water, dust and lint. But even gaskets aren't always impermeable - they only have a shelf life of 3-5 years before they need to be replaced.