Flange nuts also known as nuts with washers are mostly used for pipe connections or workpieces that need to increase the contact surface of the nut.

A metric flange nut is a nut with a wide flange on one end that acts as an integral washer.
This serves to distribute the pressure of the nut over the part being fastened, reducing the chance of damage to the part and making it less likely to loosen due to uneven tightening surfaces. These nuts are mostly hexagonal in shape and are made of hardened steel, stainless steel, usually plated with zinc.