Mine explained why I liked my undergrad degree, and why I'd chosen the topic of research for the PhD. Say what it is that you find so interesting about it, and possibly more importantly, why you think it matters. I had two interviews (and got offered both PhDs), and both really wanted to know why I thought the research was relevant. You could be the greatest researcher ever, with a great mind, dirve, and work ethic, but if you can't see the point to your research I doubt they'd be interested - and maybe after a while your interest would wane as well.
Also, go into why you chose their uni or department a little, although don't get too sycophantic. Perhaps you could get someone to check your statement - if you're still at uni, there should be a careers service or similar. Or even a current boss if you're working in a related field. And good luck!