We need an automated way to determine when writers who create accounts as minors should be granted full access to all features on our site. Date of birth is the value that determines what features are available to a user at any given time. We've put a mechanism in place that will automatically grant full social access to a restricted user on their 18th birthday. Without a field that specifies exact date of birth, we could not put that mechanism in place.
The alternative to asking for date of birth would have been to process account status changes manually (e.g., have a staff member change the status on the back end every time a community member wrote to us directly to tell us they've turned 18). However, we believe this is infeasible given the size of our community and the volume of requests we would expect to receive.