Actually, though some do look like grease zerks, I believe they were meant for oil. At first I thought they squeezed the head tubes full of grease, but read differently several years ago, somewhere. It may have old ties to racing as you mostly see these on vintage 40's to 60's racing bikes, I think. Old grease may not have had the properties we're used to today, so they had to "doctor " it occasionally to keep it working.
That's my theory anyway. Probably misguided BS.