Because a system you’re developing may have specialized modes. There’s no “today”, “yesterday” or “last week” or “q3” and other suitable shortcuts in standard date/period peekers. Another method is to use a text field which parses itself into a date or a period. E.g. “2-5” means (and/or expands into) 2023-08-02..2023-08-05. “May” means 2023-05-{01..31}. And so on.
My users always appreciated these buttons and modes because they were working in accounting and picking dates from that stupid standard picker was an ordeal. “ / / “ pattern is also annoying because you have to be precise with your cursor.
That said, ios picker is great, and it’s unnecessary to replace it. But (1) it’s not the only useful mode of operation, (2) it wasn’t always great on all platforms, and (3) html attributes usually suck at describing what patterns and use cases you want and compatibility among browsers is a minefield. I mean not only dates here, also numbers and ~numeric fields.