As others have said, it's partially a fraud prevention measure - amusingly, I have caught two people committing fraud, when I was brought in as a temporary vacation replacement in two different companies. One was the nephew of the CEO. The other was the actual CEO... In the latter, the poor accountant had been bullied for years into keeping two sets of books and, when I walked in to ask about what looked like a couple of instances of what I (at that point) assumed to be accidental double billings of clients, burst into tears, confessed the whole thing, and begged me not to tell anyone else that she was the source, as she was terrified of the CEO. So I spent the rest of the time the CEO was away, collecting evidence that could possibly have been discovered without the accountant's help, which I turned over to the board a couple days before the CEO returned. The board, as it turns out, had already had their suspicions: they had forced the CEO to hire me expressly so that she couldn't claim she was unable to take leave. They coulda warned a person...
But at my current employer, I think the liability issue is a bigger concern - and the fact that there are pay rises set in stone, so holding onto leave is perceived as a way to inflate its value and cost the company more. We're currently required to 'make a plan' for using any leave in excess of 30 days - this can be stretched a bit by booking leave well into the following year, but eventually they will get sufficiently annoyed that they 'deem' you to have booked leave for a date of their choosing. I got caught by this once, receiving a last-minute notice that I was 'deemed' to be on leave for the period between Christmas and New Year - when the campus is normally closed down anyway. Problem was, that year I was the sole person legally authorised to fulfil a particularly time-consuming role that had to be done during those dates - so I didn't even get to take the vacation I was being deemed to have...