As this is my first time growing potatoes this may sound a stupid question but here goes why do we earth up potatoes ?
It's to keep the growin spuds covered from the light. If they get exposed to light then they turn green and potentially poisonous.
I've always just planted mine a little deeper and ridge them as I plant. Never earthed up, or really needed to... but I'm just lazy ;D
It's also because potatoes grow from the stem that grows upwards from the seed potato, so the more earth you have above the seed potato, the more new potatoes have room to grow.