Even though finger nails can be useful and desirable, they're not so good if you want to be a good piano player and progress past the very basics.
A brief explanation is that to play well, your fingers need to be curved, with the very tips of the fingers making good contact with the keys. This will help you play efficiently and smoothly, and do those complex 'contortions' that we pianists have to do!
Long nails just won't let you play well, and you'll have to flatten your fingers to avoid making clicking sounds all the time!
Anyway, here's an article that gives a more complete explanation.