If we assume that an employee’s output is directly correlated to the amount of time they spend at their desk or in the office, then it makes sense why a company would try to limit and discourage vacation and sick time in order to maximize time in the office.
On the other hand, if we assume that an employee’s output is directly correlated with anything else like effort or motivation, less calculable variables that vacillate—troughing when an employee is at their desk too long and peaking when an employee is well-rested—then it makes sense why a company would offer unlimited vacation and flexible work hours.
Also assuming that the employee is adult and professional enough to manage their own time and still meet expectations.