From Edward Feser’s brilliant volume Aquinas: A Beginner’s Guide, p. 30:
The significance of the distinction between essence and existence is indicated by another argument Aquinas gives for it. If essence and existence were not distinct, they would be identical; and they could be identical only in “something whose quiddity is its very act of existing … such that it would be subsistent in itself” (DEE 4). That is to say, something whose essence is its existence would depend on nothing else (e.g. matter) for its existence, since it would just be existence or being. But there could only possibly be one such thing, for there would be no way in principle to distinguish more than one. We could not coherently appeal to some unique form one such thing has to distinguish it from others of its kind, “because then it would not be simply an act of existing, but an act of existing plus this certain form”; nor could we associate it with some particular parcel of matter, “because them it would not be subsistent existence, but material existence,” that is, dependent on matter for its being (DEE 4). In fact there is, in Aquinas’s view, a being in whom essence and existence are identical, namely God; and the identity of his essence and his existence entails (among other things) that God is a necessary being, one that cannot possibly non exist. But all of this shows that in everything other than God, essence and existence must be distinct. For in the case of material objects (for example) there is more than one member of each kind, and none of them exists in a necessary way but only contingently; and this would not be so if essence and existence were in these things identical.