Why do human beings have laws?

As in most things, there is wisdom to be found in the works of the Angelic Doctor:

Man has a natural aptitude for virtue; but the perfection of virtue must be acquired by man by means of some kind of training. Thus we observe that man is helped by industry in his necessities, for instance, in food and clothing. Certain beginnings of these he has from nature, viz. his reason and his hands; but he has not the full complement, as other animals have, to whom nature has given sufficiency of clothing and food. Now it is difficult to see how man could suffice for himself in the matter of this training: since the perfection of virtue consists chiefly in withdrawing man from undue pleasures, to which above all man is inclined, and especially the young, who are more capable of being trained.

Consequently a man needs to receive this training from another, whereby to arrive at the perfection of virtue. And as to those young people who are inclined to acts of virtue, by their good natural disposition, or by custom, or rather by the gift of God, paternal training suffices, which is by admonitions. But since some are found to be depraved, and prone to vice, and not easily amenable to words, it was necessary for such to be restrained from evil by force and fear, in order that, at least, they might desist from evil-doing, and leave others in peace, and that they themselves, by being habituated in this way, might be brought to do willingly what hitherto they did from fear, and thus become virtuous. Now this kind of training, which compels through fear of punishment, is the discipline of laws. Therefore in order that man might have peace and virtue, it was necessary for laws to be framed: for, as the Philosopher [Aristotle] says (Politics. i, 2), "as man is the most noble of animals if he be perfect in virtue, so is he the lowest of all, if he be severed from law and righteousness"; because man can use his reason to devise means of satisfying his lusts and evil passions, which other animals are unable to do.

—St. Thomas Aquinas (1225-1274), Summa Theologica/Treatise on Law, I, II, Q. 95, Art. 1, translated by the Fathers of the English Dominican Province (Benzinger Bros., 1947).