You're checking out on an e-commerce site for the tenth time this month. You start typing your shipping address... and your browser suggests the wrong address. You're trying to enter your work email, and it keeps suggesting your personal email. You give up and type everything manually, again.
Or maybe you're someone with a cognitive disability who struggles to remember your address. Your browser could help you fill in forms automatically - but only if the website tells the browser what kind of information each field expects.