There are two ways to look at this.
First, it is ethically important to control these costs. Controlling workers compensation costs is done by making one's workplace safer. It is ethically important to make the working environment as safe as possible so as not to expose one's employees to injury or illness.
Second, it is pragmatically and economically important. Workers' compensation insurance can be very expensive. It becomes more expensive if the firm has many injuries and illnesses that are caused by work. Therefore, it is important to control these costs simply as a way of making sure that the firm is not paying out huge sums of money on something that is not related to improving productivity.