It’s not just anti-fraud. If you force someone to take a vacation, you force the organization to be able to function without them. This prevents a situation where only a single person is able to make something critical work.
The rest of the days might be lost, transferred into next year’s vacation with a cutoff (like until end of Q1 next year or end of the year otherwise they are lost), or with no cutoff and they stack up forever.
If the PTO is not taken, the company can be fined by the state, so this is taken rather seriously.
And if you delay two weeks to next year that means 5 mandatory weeks that year.
And: it is not just in theory. It is the law and both parts can get in trouble if you don't follow it.
It also forces people to communicate details somewhat.
Although around Old Street the buses don’t really go above walking speed.