Sometimes when you are on your device and you want to zoom in on an app to look at it's UI. You can do it easily by enabling zoom. Go to Settings app > General > Accessibility > Accessibility Shortcut (at the bottom) and choose Zoom.
You can then triple-tap the home button to toggle zoom mode on/off. When zoom is on, drag three fingers to pan the screen and double-tap three fingers and drag to change zoom level.