Why does Colonel Saito visit the prison camp in the film, The Bridge on the River Kwai?

Captain Saito arrives at the camp and announces his intention to build a bridge over the River Kwai, and to tell them that he expects all men, including the officers, to work. After warning the men to work hard, Saito tells them that Japan cannot feed idle men, and if they try to escape, they will die. They have one day of rest before they begin working. He tells them, "Be happy in your work."