Imagine a situation you are in the open wearing cotton trousers and jacket and it starts raining. You can either end up dripping wet or you can take out properly packed waterproof suit out of your backpack and stay dry. Getting dressed is then a matter of seconds and you remain dry.
Waterproof suits are either cheaper – rubbered, suitable for rather shorter use because they are not breathable; or made of membrane fabrics which do not let wet in, but do let it out.
Most of the suits are two-piece – trousers and jacket.