I could never quite figure this out. It seems to be actually more hassle, in terms of paperwork and reporting, to pay for your employee to park a work vehicle during work hours while on a job made at your request (which is entirely a business expense) than it is to pay for them to have a reserved city centre parking space (which could easily have a significant personal benefit component).
The other weird thing is: I don't even think the employee has to use it! Would their SO find such an arrangement useful? Well, there you go: they could use it instead! There really don't appear to be any restrictions on this stuff. It's extraordinary.
On the flip side: this does mean, if you're negotiating a new job, maybe you could think about trying to get them to pay for a parking space for you, if that would be useful. Because it's entirely a business expense and so cheaper than raising the salary.