Manual labor is directed to four things.
First and principally to obtain food; wherefore it was said to the first man (Genesis 3:19): "In the sweat of thy face shalt thou eat bread," and it is written (Psalm 127:2): "For thou shalt eat the labors of thy hands."
Secondly, it is directed to the removal of idleness whence arise many evils; hence it is written (Sirach 33:28,29): "Send" thy slave "to work, that he be not idle, for idleness hath taught much evil."
Thirdly, it is directed to the curbing of concupiscence, inasmuch as it is a means of afflicting the body; hence it is written (2 Corinthians 6:5-6): "In labors, in watchings, in fastings, in chastity."
Fourthly, it is directed to almsgiving, wherefore it is written (Ephesians 4:28): "He that stole, let him now steal no more; but rather let him labor, working with his hands the thing which is good, that he may have something to give to him that suffereth need."
This was a big issue at the time, of course; the rise of universities and the mendicant orders put front and center the question of how and in what way manual labor should be required of everyone, so it was important to determine what the proper role for manual labor should be in a civilized society. St. Thomas's answer on that point, which is a compromise position, is that it is a matter of natural law that people who are able should do the work required to sustain themselves rather than live off of others; but that this is a higher-level precept, i.e., it admits of individual exceptions where there is some greater benefit to the common good of everyone -- Thomas's major example, of course, is at least partial exemption of teachers from manual labor so that they can be less fettered in providing the good of education. In any case, one has to determine what the rational ends of manual labor are, and this is his answer: it provides the means of supporting oneself, it gives one something constructive to do, it develops habits of self-discipline, and it gives you the means of helping others who need help. And Aquinas, of course, takes a hard line on the latter, a high standard against which most of us would fail: if you don't need it to support yourself you are obligated to use it for the good of others (although he allows that there may be a great many different ways to do this).