Serrated flange nuts have a wide flange that acts as an integrated washer. This distributes the pressure of the nut over the part being secured, reducing the chance of damage to the part and making it less likely to loosen as a result of an uneven fastening surface.
The serrated flange provides a locking action. Because of the serrations they cannot be used with a washer or on surfaces that must not be scratched. The serrations help in preventing the vibration of the nut from moving the fastener, thus maintaining the holding power of the nut.