The porridge oats stick to the inside of the spoon like glue when you are trying to mix and stir it. It also doesn't get it off the bottom of the pot as well when stirring. The spurtle usually has a fairly flat bottom and is easy to wipe off on the side of the pot 'tween stirs. When the porridge is ready you can tip it from the pot using the spurtle as a sort of lever, and wipe off the edge with it. No serving spoon required, and the pot is often deeper than a spoon.
I've made mine from a not-quite-square bit of pine batten and rounded the corners, so it's even easier to wipe off, and easier to scrape down the sides for the last delicous bits!
But Kepis has shamed me, I feel a bit inspired to try to decorate the top a bit.