[Dopey113]

ok I could google it, but would rather hear from ppl who actually know, and is it really necessary?

[Goneterseed]

The potatoes don't grow from the root of the plant, they grow from the lower stem. The more you earth the stem the more potatoes you get, or that's the theory at least.

[mumofstig]

AS Gts has said
Also the deeper earth layer should  prevent the topmost potatoes from greening from sunlight....alternately grow under thick mulch.
Potatoes grown in ridges are easier to harvest than those planted deeply and not earthed up.