In Mail you are presented with a navigation "drawer" on the left that is always open and cannot be closed. This drawer is all the way on the left and includes all your email accounts and folders for that account. Now at first you may be thinking that, "well, I use those all the time." Not to worry you can click on the navicon button and keep it open, but you're also probably a developer or at least a "power user." For most users they spend their time in their inbox and the rest is just a distraction and takes away from the point of a mail app, the email and content of those emails. This is where the focus should be and those secondary items should be hidden from a default view. A good example of this is looking at Google Gmail vs Google Inbox. Gmail is cluttered with tons of different places you can go within the app and moves your focus away from the heart of the app, email. Inbox, on the other hand, is much more clean and simplifies the mail experience at every corner (hiding secondarly navigation items by default, auto categorization groupings, marking your mail items as done or snoozed, etc). This allows the focus to be on mail. Auto hiding the nav drawer on the left is one step in that direction.
