[M12 1_Large]

The bottom of the hex nut is a brim, which is called a "flange"
Flange part acts as washer
Compared to the standard hex nuts and washers, the use of flange nuts instead of washers reduces the hassle of incorporating washers and improves work efficiency.
The original purpose is that many users use the nut to stabilize the seat and the appearance is beautiful.
Easy to use and convenient