A square nut is a four-sided nut .A square nut is a four-sided nut. Square nuts are used along with flat washers in order to avoid damage from its sharp edges and increase the strength of the fastener.
The main benefits of using these nuts are they are tightened easily by gripping two sides,  work well in tight spaces by using needle-nosed pliers, work well in blind spots using pliers or a wrench, and can be a quick gauge to measure the position of the nut.