In certain circumstances, an employer and employee (many times, but not always, a senior executive) will enter into a written employment agreement as a part of the employment relationship. These agreements will describe the employees job duties. They will also, importantly, address such questions as base compensation, term, change of control, acceleration of equity, and severance.
An employment relationship is more likely to succeed when the parties expectations are well-defined. And if the relationship is failing, an employment agreement can set the terms for a smooth and amicable separation.