These emails are sent out whenever someone logs into your Gmail account, which is useful for when you are viewing the Gmail at the same time, or have a push notification service(like on your cell phone for notifications, which most people do). This service is asynchronous and ALL logged in users will see the email, even if one user sends it to trash(it would require a refresh on all users to see the message has been deleted).
So a common workflow for this is you're checking your emails when you are notified(instantly in the web and instantly by notification) that someone has logged into your account on a new device(or attempted too). If this happens you now INSTANTLY know in a way the hacker can't prevent. This gives you a small windows of opportunity yourself to stop the hacker while your Oath token is still good and you can lock the account(or if a bit too slow, start a recovery ticket). In asynchronous applications like gmail, yahoo mail, outlook, and many other providers where a login looks... fish-e... this can give the user time to fix the problem.
Now this may sound like "Well why doesn't the hacker just change the login details then?" Sometimes this happens, but if ready for it a user should be able to start the damage mitigation process themselves, and/or seek help to minimize the damage that does happen.