All of us need a break from our jobs now and then, even the true workaholics. The time we get to rest, recharge, and enjoy ourselves isn't just crucial for helping us do better at work in the long run, but also something we need for our own wellbeing, full stop. Vacation time is a part of having a quality of life.
Unfortunately, a lot of employers do not seem to share that view. They believe that their employees should be willing to drop their PTO whenever they feel like they need it, and they act accordingly. While there are occasionally valid reasons to deny time off, in many cases this is the sign of a company that is not good to work for. If they are willing to deny their workers the vacation days that they should technically be allowed, they are probably going to mistreat them in other ways, too.