Few Excerpts from the blog: While a birthday is easy to key in, some people might still prefer picking out the date entities from a given set. While a dropdown for the day, month, and year fields seems to be a pretty straight forward method, the scrolling makes the experience unbearable (Imagine trying to select your birth year)

So designing a picker that is bound with the input and opens the appropriate day, month, or year dialog automatically based on what the user is currently focussed on is a much better way. An advanced user might prefer to key it in and someone in a hurry can select without having to navigate and scroll through multiple dropdowns.