During WWII the military tried to reduce weight and bulk of many tools by adapting them to be used with square drive bars and extensions. This was especially true of aircraft tools. The leadership assumed that there would be square drive bars and ratchets to provide handles for these tools, since the drive tools were to be part of a complete tool set anyway. Thus, many AN-series tools follow this design profile. It makes a lot of sense if you think about it.
I have seen three types of square drive spanners (American term, not British - which would describe regular "wrenches" as we Americans call them), Pin spanners, like this one, Hook spanners, which have a flat-faced hook instead of the pin, and pin face spanners, which have pins on the sides of the tool that engage holes drilled in the face of a "nut" like the gland nut of a hydraulic cylinder.