Each goddess is used as enhance material.
they give +1 level if used incorrectly or +3 level if used correctly.
you use then correctly by enhancing units with same property (Anu, melee unist; Danu, archer; Tartu, magic unit);
Also epona is for selling, her price is 100k luna.
For the info, go for Quest whim of goddess and look for the info on "Unit of goddess" (how google translated for me), below quest info, on the same japanese wiki.