The only way the field would be used is if a person decided to use a different piece of software that wants a birthdate.
It leave the door open for this to happen. A malicious software may not advertise that it is harvesting your demographic information. Now that this is in place it's one more thing we have to worry about when evaluating software. There is absolutely no reason to be storing PII in a centralized spot where anything and everything can request it. If I want an app to have any of that shit I'll enter it on a case by case basis.
You can say "well don't put it in there" but what happens when big monopolistic corporations start requiring it to use some service of theirs that you don't have an alternative for? Now I have to maintain a separate PC for that shit? Fuck that.