Firstly, knowing what your customers are thinking about their workplace gives you a lot of useful insights. Secondly, acting on employee feedback will eventually result in happier staff members, which in turn boosts productivity.
Apart from job satisfaction an employee survey may also reveal useful insights about management. Managers directly affect employees either positively or negatively. Use employee feedback to identify development opportunities for specific managers or for the entire management team.
Employee satisfaction surveys can also be used to assess team performance and identify opportunities there. Remember: great teams accomplish great things!