You’re talking about slightly different things. You’re right that when the UI opens it’s always going to be on a default date, if you don’t set one it’ll be today.
But if you don’t set one then the value of the HTML element will be null before it’s selected. So if you add a required attribute then you can make sure that your users use the date picker (or leave in frustration after failing to do so) before submitting the form.
If you set a default value using the value attribute, then that becomes both the HTML default value and the UI default. IMO the spec is missing a way to just set the UI default, so that the field is default empty but interacting puts it at a sensible starting point.
There’s an MDN example here where the date starts empty and is required, if you try submitting the form you’ll see there’s no default for the HTML element: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/in...