This is the place where the date picker does not help the user at all. It's easier to type the, presumably memorized, date, than to look it up in the calendar no matter how nice and handy the calendar is. Sure it does solve validation problem. Or maybe not correctly, don't ask about locales and date adjustments.
Even if, that does not help. When asked for a birthday I need validation of the date in country when I was born, not the country I'm in, not the locale I'm using for display and not the locale server/requesting company is in.
Obviously, but additionally, providing validation on the frontend can help UX a lot.
Doing that can provide much quicker feedback compared to an error thrown at the user only after submitting a form, which can get especially annoying if the latter loses (some of) its values due to submission.
And one solution for that problem can be using a native picker.
I do understand what "client-side" validation is, but I wish it had a different name, because people think they can just validate client-side and they do not bother doing it on the server... for some reason, I do not know. It should be obvious though, right? Yet it is not.
