In some places, that policy is in place to encourage employees to actually take time off. If it wasn't use-it-or-lose-it, some people would never take leave.
We always hear this, but it doesn't really make sense. First, a firm that actually wanted employees to take vacation could just make a policy by which a certain percentage of banked time would be automatically scheduled by HR after so long a period of banking. Second, under the "lose-it" policy, many employees don't actually use all their allotted vacation. This can happen even to those who really try to schedule it. A cynical manager can always stymie the best-considered vacation plans. I consider that this common result is actually the intent of this anti-employee policy. (At many firms; of course any particular firm may be an exception.)
No, it is so the employer doesn't need to carry the additional liability in their accounting. Vacation time is something with a monetary value that must be paid out if the employee is terminated.
