All Mail is just that...all mail in your account. Gmail uses labels for everything else, so the Inbox "folder" is just a label and moving it out of there into All mail just removes the label, it doesn't delete the copy since there is no copy. Sent mail is the same way, there is a "Sent Mail" label that gets applied to all your sent messages. Without the All Mail folder, you won't have any messages since that's where the email truly resides. The Inbox and Sent Messages are just labels that make it easier for you to manage messages.