The elongation of the parenchyma cells pushes the root tip into the soil, the calyptra covers and protects the delicate apical meristem from abrasion as it grows through the soil.

The root cap is formed by several layers of cells that envelop the root tip externally. This covering of cells, of which the outermost ones are dead, envelops and protects the growing tip very much as a thimble protects the finger.  It extends back over the root for a distance usually of about a millimetre.
As the root-cap cells mature, they become parenchyma and are constantly pushed out by the addition of new cells from within. Those on the outside next to the soil are more or less flattened by pressure from within and slough off as they rub against soil particles when the root grows deeper. The root cap produces a slimy layer of mucilage, known as mucigel that lubricate the tip. But throughout the life of the root, even if it penetrates many feet into the soil, the root cap is so constantly renewed by new cells from within that the delicate tip is actually pushed through the soil without, as it were, coming in contact with it. This explains why roots can penetrate even into stiff clay soils without buckling.