What's the advantage of that?
A. If you don’t know the DOB at time of booking.
B. If you want to book an unaccompanied minor, you can book them as an adult and change the date later.
C. If you want to book a seat for an infant, many websites don’t give you that option, so you book them as a child and change the date later.
There were some more cases I’ve done it, but these are the ones I remember at the moment.