I think that Ford is saying that the assembly line is good in at least a couple of ways.
First, he is saying that it is easier on the workers. One of the principles of the assembly, he says, is that no worker will have to take more than one step to do his job and that no worker will have to stoop. This would make the work (according to Ford) less strenuous.
Second, he is saying that the assembly line is economically useful. He talks many times in this book about how this part or that part or the whole car could be made many times faster once the new assembly line system was in place. This made the workers much more productive and helped Ford's company make more money.