The primary advantage of waterjet cutting is that not heat is generated in the process. This the material which is cut does not undergo any transformation in the surface due to the process. This is different from other mechanical cutting processes where heat is generated and a coolant is used.
The water jet can be incorporated into a CNC system which can cut the material according to a preset pattern.
Sometimes, an abrasive is added to the water in order to cut hard substances such as granite.
Video of a WaterJet