Rather than look at the question from a statistical viewpoint, let’s look at it from a more subjective human perspective  . . .  

Firstly, a healthy employee is a better performer, creating a more efficient and effective workforce overall.   A physically and mentally fit workforce means reduced sickness absence, which could save employers millions of pounds each year.  You couldn’t ask for a better incentive!

Secondly, from a company’s perspective, employees are an enormous expense.  The bottom line is that any company wants to retain their best and brightest.  In this regard, Occupational Health and HSE are valuable retention tools.  Who wouldn’t want to work for a company that demonstrably shows it’s commitment to employee wellbeing and safety?

Thirdly, it’s the right thing to do.  From a moral and ethical standpoint, corporations have a responsibility to provide a safe and secure environment for their employees.  In any employment contract there is an implicit understanding that the employee will do their best for the company, and the company will do it’s best for the employee.  Proactive occupational health and HSE policies speak eloquently of management’s commitment to the welfare of their employees. 

So the answer to the original question, why do we need occupational health, has to simply be, because it makes sense.