Picture a form that asks for "First name" and "Last name". A user from Hungary types "Nagy" in the first box and "Anna" in the second, because in Hungarian the family name comes first. A week later your app sends her an email that starts "Hi Nagy".
Or a user from Indonesia has one name, and only one. Your form requires both fields, so they type their name twice, or put a full stop in the second box, and now your database says their surname is ".".
A name feels like the most basic thing you could ask a user for. But the "first name, last name" form is built on one culture's idea of a name, and plenty of people's names don't fit it.