Here is a classic explanation from Ludwig von Mises in his magnum opus titled “Liberalism”. The full text of this work can be found on Mises Institute.

The idea of freedom has become so ingrained in all of us that for a long time no one dared to call it into question. People were accustomed always to speaking of freedom only with the greatest of reverence; it remained for Lenin to call it a “bourgeois prejudice.” Although the fact is often forgotten today, all this is an achievement of liberalism. The very name of liberalism is derived from freedom, and the name of the party in opposition to the liberals (both designations arose in the Spanish constitutional struggles of the first decades of the nineteenth century) was originally the “servile.”

Before the rise of liberalism even high-minded philosophers, founders of religions, clerics animated by the best of intentions, and statesmen who genuinely loved their people, viewed the thralldom of a part of the human race as a just, generally useful, and downright beneficial institution. Some men and peoples are, it was thought, destined by nature for freedom, and others for bondage. And it was not only the masters who thought so, but the greater number of the slaves as well. They put up with their servitude, not only because they had to yield to the superior force of the masters, but also because they found some good in it: the slave is relieved of concern for securing his daily bread, for the master is obliged to provide him with the necessities of life. When liberalism set out, in the eighteenth and the first half of the nineteenth century, to abolish the serfdom and subjection of the peasant population in Europe and the slavery of the Negroes in the overseas colonies, not a few sincere humanitarians declared themselves in opposition. Unfree laborers are used to their bondage and do not feel it as an evil. They are not ready for freedom and would not know how to make use of it. The discontinuation of the master’s care would be very harmful to them. They would not be capable of managing their affairs in such a way as always to provide more than just the bare necessities of life, and they would soon fall into want and misery. Emancipation would thus not only fail to gain for them anything of real value, but would seriously impair their material well-being.

What was astonishing was that one could hear these views expressed even by many of the slaves whom one questioned. In order to counter such opinions, many liberals believed it necessary to represent as the general rule and even on occasion to depict in an exaggerated manner the exceptional cases in which serfs and slaves had been cruelly abused. But these excesses were by no means the rule. There were, of course, isolated instances of abuse, and the fact that there were such cases was an additional reason for the abolition of this system. As a rule, however, the treatment of bondsmen by their masters was humane and mild.

When those who recommended the abolition of involuntary servitude on general humanitarian grounds were told that the retention of the system was also in the interest of the enslaved, they knew of nothing to say in rejoinder. For against this objection in favor of slavery there is only one argument that can and did refute all others-namely, that free labor is incomparably more productive than slave labor. The slave has no interest in exerting himself fully. He works only as much and as zealously as is necessary to escape the punishment attaching to failure to perform the minimum. The free worker, on the other hand, knows that the more his labor accomplishes, the more he will be paid. He exerts himself to the full in order to raise his income. One has only to compare the demands placed on the worker by the tending of a modern tractor with the relatively small expenditure of intelligence, strength, and industry that just two generations ago was deemed sufficient for the enthralled ploughmen of Russia. Only free labor can accomplish what must be demanded of the modern industrial worker.

Muddleheaded babblers may therefore argue interminably over whether all men are destined for freedom and are as yet ready for it. They may go on contending that there are races and peoples for whom Nature has prescribed a life of servitude and that the master races have the duty of keeping the rest of mankind in bondage. The liberal will not oppose their arguments in any way because his reasoning in favor of freedom for all, without distinction, is of an entirely different kind. We liberals do not assert that God or Nature meant all men to be free, because we are not instructed in the designs of God and of Nature, and we avoid, on principle, drawing God and Nature into a dispute over mundane questions. What we maintain is only that a system based on freedom for all workers warrants the greatest productivity of human labor and is therefore in the interest of all the inhabitants of the earth. We attack involuntary servitude, not in spite of the fact that it is advantageous to the “masters,” but because we are convinced that, in the last analysis, it hurts the interests of all members of human society, including the “masters.” If mankind had adhered to the practice of keeping the whole or even a part of the labor force in bondage, the magnificent economic developments of the last hundred and fifty years would not have been possible. We would have no railroads, no automobiles, no airplanes, no steamships, no electric light and power, no chemical industry, just as the ancient Greeks and Romans, with all their genius, were without these things. It suffices merely to mention this for everyone to realize that even the former masters of slaves or serfs have every reason to be satisfied with the course of events after the abolition of involuntary servitude. The European worker today lives under more favorable and more agreeable outward circumstances than the pharaoh of Egypt once did, in spite of the fact that the pharaoh commanded thousands of slaves, while the worker has nothing, to depend on but the strength and skill of his hands. If a nabob of yore could be placed in the circumstances in which a common man lives today, he would declare without hesitation that his life had been a beggarly one in comparison with the life that even a man of moderate means can lead at present.

This is the fruit of free labor. It is able to create more wealth for everyone than slave labor once provided for the masters.