I think it's so you can give it 1/4 turn, 1/8 turn etc.
Years ago I bought 100 of those when they were dirt cheap, gave them to everyone I did a saw for as a reminder to tune their saw.
My square STIHL "screwdrivers" have a flag, figuring out partial turns down to 1/16 of a turn and full turns works fine. (I also have the longer version of the below image, though haven't found that to be an advantage.)
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