Why would birds develop backward knees?

The other day I was thinking about how weird birds' legs are and how their knees are backwards compared to people knees. Why would birds develop backward knees?

Actually birds’ knees bend the same direction as our knees do. However, the knee is usually hidden under feathers close to the body.

Most birds actually walk on their toes. The joint that you are seeing “bend backward” is the birds’ ankle. And everything under the ankle is the foot. Basically, the bird has a long skinny foot with long skinny toes, and their knee, the one that is covered up with feathers, bends the same way as ours.

Source: Wikipedia Bird anatomy http://en.wikipedia.org/wiki/Bird_anatomy