Vet school is highly oversubscribed, they can afford to be selective, that is why they command high grades and some places, work experience. Having good grades and lots of work experience puts you ahead of most other applicants who don't have the grades and don't have the experience. The university is more likely to interview someone with work experience and good grades, over someone with good grades but no work experience.
And, as said earlier, work experience makes sure you really want to be a vet, it gives you stuff to talk about in your interview which you can comfortably do, rather than harder academic questions. It shows commitment, the vet course costs the university a lot of money to run, they don't want people pulling out because it costs them money, they want people who can stick the course. And it does help you when you get in when handling the animals.