Amazon order emails (confirmations and shipping notices) don't say what was actually ordered, just an order number and maybe a price. So if you buy a lot of stuff on Amazon and/or don't remember what you ordered, you have to click through to find it.
Also you can't search your past emails to see when/where something was purchased; you have to remember that you bought it on Amazon, and then search there. Much more convenient to have it in your email archive along with receipts from other sources.
That's not true (for all of Amazon at least), I ordered a phone case:
Subject: Your Amazon.se order of "Spigen Liquid Air Fodral..."
Has arriving, delivery option, place it will be sent to, delivery preference, order total, payment method, name+picture+link of the product(s) and order number
And then shipping notice:
Subject: Your Amazon.se order of "Spigen Liquid Air Fodral..." has been dispatched!
and finally:
Subject: Arriving today: Spigen Liquid Air Fodral...
With order number and name+picture+link of the product(s)
Also you can't search your past emails to see when/where something was purchased; you have to remember that you bought it on Amazon, and then search there. Much more convenient to have it in your email archive along with receipts from other sources.