The term "lock nut" or "locknut" can refer to two different pieces of hardware:

The one on the left is a nut with a nylon insert, which grips the thread of the bolts to prevent the nut from undoing itself through vibration. The one on the right, often called a conduit locknut, is used for, well, conduit.

This latter type of locknut is often located in tricky-to-reach areas of junction boxes. Klein Tools thus designed their Locknut Wrench with offset bends to make access easier. To further increase access, they made the tool as slim as possible.

The wrenches are sold as a set of three, covering 1/2", 3/4" and 1" sizes, color-coded for easy identification. Embedded magnets hold the trio together. It's great design, and affordable for tradespeople at $15 for the set.