It is also used in situations where a standard nut would not fit.It is commonly used as a type of locknut, where it is "jammed" up against a standard nut to lock the two in place. Lock Nut, 5/8"-11, Steel, Zinc Plated, 5 PK.
USS flat washers have a large, wider body. Flat Washer, Fits Bolt Size #10 ,Steel Chrome Plated Finish, 10 PK. SAE flat washers are machined to a tighter specification and have a smaller outside diameter.