The iPhone Big Screen concept by Tristan Dalay, enhances the viewing capabilities of the 4 inches screen and becomes good to watch movies.
When using the Big Screen the touch screen functionality is disabled, but the design provide access to the volume and home buttons.
via Yankodesign
Looks like it is a tool that is added to make the iPhones creen look larger.
There’s also Samsung Galaxy S4 Concept that you can see 🙂