Most full line hand tool companies sell a pipe plug socket set from 3/16 to 3/8. I can think of many applications for the 3/8, but what are the smaller ones used on?
I've run into the smaller sizes on 1/8 & 1/4 NPT pipe plugs on truck air systems and such. Not super common, but they do have them on some other applications.
A 3/8 in 3/8 drive & 1/2 in 1/2 drive will cover 99% of what most will run into.
Cat 3406e motors have a pipe plug going into an oil passage on the lower right side of the block that takes a 5/16" square male pipe plug socket. Usually 1/4, 3/8, and 1/2" get you by.
I just had to use a female 9/32 to shut off the hot and cold water lines under a sink. The shut valve doesn't have traditional handles, only a male square drive..... presumably their idea of a "tamper resistant" design?
I had started with my 8-point 1/4 and 5/16, but nope.....