How could we attain knowledge if there were no stable object? And how could the world be what it is, if it were not grounded on stable principles? It is common knowledge that Plato answers both these questions, an epistemological and an ontological one, by his theory of the Forms.
From an epistemological viewpoint, the Forms constitute the objects of true knowledge. From an ontological point of view, they are the principles that underlie the order of the universe.