It really doesn't. You see plenty of various items across the cultures that appear without any practical use whatsoever. The best example is probably carving reliefs into stone - sure, sometimes they are there to tell a story, but most often, they serve as a status symbol or a simple decoration.
Warfare is no different - give a soldier a partially wooden weapon, and he will soon carve stuff into the wood.
Spoiler: 4th century Roman
Spoiler: WW1
Rank insignia is a possibility, but placing that on the business end of a spear has its problems, and Occam's razor says we should lean towards the simplest explanation - soldiers wanted to make their stabby stick look nice. And that sometimes makes you do wildly impractical things:
Spoiler: Germany, 1560