They make you a better all-around worker
According to The New York Times, when accounting firm Ernst & Young looked at the vacation habits of their workers, they found that, in general, the more vacation someone took, the better their year-end performance review. Other studies show that workers who used their paid time off were more effective and dedicated overall, and less prone to burnout.
De Graaf sums up nicely the benefit employers receive from giving workers plenty of vacation, saying, "Workers who take vacations are less likely to have health problems, which decreases absenteeism. They're nearly twice as likely to feel happy about their workplace and work, and studies show clear connections between happiness and productivity."
Basically, your boss needs you to go away. It's the best thing for both of you.