Right angle Zerk fitting (grease fitting). I have a similar one that just has a t-handle rather than a square-drive hole for a ratchet.
You learn something new everyday..
I was wondering too and I was going to post a smartass response that the purpose was to look pretty or fill some obscure place on your socket tray.
But you beat me to it.

i did a quick search but I'm on my phone....
Does anyone make a general set of the common sizes for these?
EDIT: looks like APEX makes a bunch...still looking for a set....
Right angle Zerk fitting (grease fitting). I have a similar one that just has a t-handle rather than a square-drive hole for a ratchet.