I have not used that specific fitting, but it looks like a version of what I always knew as the "button head grease fitting". Usually used where high pressure is not wanted.
I've got a few of these in my fitting collection. Used on the dozer track bearings where you don't want to blow out the seals.
Alemite 1823-1 Giant Button Head Fitting, 1-1/4" OAL, 7/8" Hex, 1/2" Shank Length,