Water contains calcium and magnesium salts. When water is heated in your coffee maker, calcium and magnesium break down and precipitate to form limescale, or scale. This affects the boilers, pumps and any interior parts that carry water. In the areas where the water sits still for a while, such as the boiler, scale builds up.
The build-up of limescale over time will slow water flow, reduce boiler capacity, create heating problems and eventually cause blockages.
To prevent all this you need to descale your coffee machine.