Should sales managers focus more on which, existing customers or bringing new customers? how much percentage they should apply to each and why?

Basically, if a manager only focus on the existing customers, there would be a possibility that the profit of a company would going to be in a cruicial since it's just like you're waiting for customer to come in. I mean, why would need a staff if you're just going to wait for customer to come in? what the sense of having a business? 

A sales manager should have to focus not only on the existing but for the future customers or for the possible prospects for the company to gain or to grow.