When you press back button on device, app saves current text and closes the application. No need to extra save action. Next opening, saved text will be on the display.
When you select a part of text or all of them, a menu will appear. By this menu, you can copy and cut selected text.
If you want to clear all text, please select all and than cut. =) There is no "delete all" button. Because this app should be easy as possible.