I am not a manager, so I am merely speculating here, but I suspect measuring productivity is important in determining which employees are of most value to the company (who is pulling their weight, so to speak), particularly for example in the cases of raises/promotions or redundancies. Also, different people will have different skills and preferences, so if a team member shows a particular aptitude for a set of tasks and is generally more productive at doing those, it may be better to give more tasks of that style to that team member to boost productivity.