For Plato, the forms transcend the realm of particulars. The forms are more real than the transitory, mutable objects which participate in or imitate them in the physical world. Plato also makes it clear the the form of the Good is the highest form. Thus the form of the Good is the first principle for Socrates. The other forms participate in the form of the Good, so the Good functions as the underlying substance of all that is. Just as shadows exist only because of the light of the sun, the world exists because of the Good.
However, because the form of the Good is the underlying substance of all that is, it appears to function as the form of Being within Plato's theory. Hence subsequent philosophers, particularly the scholastics, would modify the theory to making Being the first principle. On this account, Being is the underlying substance of all that is, and the True, the Good, and the Beautiful are different aspects of Being.
Furthermore, the True, the Good, and the Beautiful, as aspects of Being, are intricately related to beings such as ourselves. Human beings are capable of thinking, acting, and making. These activities correspond to the transcendentals, which constitute their proper ends. For philosophers, logic, ethics, and aesthetics are the fields of inquiry that relate to these three transcendentals. Consequently, metaphysical inquiry can tell us about more than just what ultimate reality is like. It also shows us more fully who we are as beings.