In many jurisdictions, there's a statutory minimum amount of annual leave; you can take less but then the difference must be paid in lieu. (Versus if you give 30d, 18d is minimum, and employee takes 20d, then you don't have to do anything. Obviously could have a company policy of paying it if you want.)

So, a more cynical take on mandatory time off, and something that absolutely does happen, is that they'd just rather not have the work than have to pay extra for it.

Also common is not rolling over into the next year, so use it or lose it.