That’s what Apple does to some extend. “Siri found in mail” contacts and events. Maybe some more.
Siri pulls from Messages for those things as well, and will pull spelling and other suggestions out of Safari browsing.
Yes. A better question would be “why does google itself need this information when it’s just a client side UI feature?”
Because... Google's business model. Your ability to see your purchase history on a Google page is neat, but it seems a safe bet that the real reason they want your purchase history is because their profile about you as a set of eyeballs becomes more valuable allowing them to charge higher rates on ads. Notice that they only claimed to have stopped scraping gmail for ad targeting purposes, not that they stopped scraping period.
Gmail's primary UI is a web browser. How... how do you think that _actually_ might work?