Thomas Aquinas, Compendium of Theology

We showed above that God has brought things into existence, not through any necessity of His nature, but by His intellect and will. Any agent that works in this way, acts for an end: the end is a principle for the operative intellect. Accordingly everything that is made by God necessarily exists for an end.

Moreover, things were produced by God in a supremely excellent way; for the most perfect Being does everything in the most perfect way. But it is better for a thing to be made for an end than to be made without the intention of achieving an end; for the goodness that is in things which are made comes from their end. Hence things were made by God for an end.

An indication of this is seen in effects produced by nature. None of them is in vain, all are for an end. But it is absurd to say that things produced by nature are in better order than is the very constituting of nature by the first Agent, since the entire order of nature is derived from the latter. Clearly, therefore, things produced by God exist for an end.