Here's the function of mine:

UtiliKilts... no kilt pin... looks pretty dumb on the thin apron. I do, however, decorate the side pocket with some bulkier pins and cap badges. They look rugged.

Pittsburgh Kilt... no kilt pin... no function really since it's lays perfectly. If I had a solid, and not a camo, I might go with something to add some shine.

Poly/Viscose kilts... heavy kilt pin... keeps the thinner material apron from flying-up in winds. These are the only kilts that I own that I wear the kilt pin for function.

Heavy Irish National from House of Edgar... Irish kilt pin... decoration only.

For me, the pin is there for a little bit of "bling" on an otherwise straight-up garment apron. With the sporran, they add some character to the kilt. The buckle, sporran and pin all work in conjunction to add a theme or mood to the whole ensemble.