The screen button works like the keyboard in that it takes it’s picture when you release the button – not when you press it. You can launch the camera app, hold the software button down with your finger, hold your arm out to get just the right picture and release your finger. Great when you want to take a picture with yourself in it… no hunting around for the button.
Another feature is that the iPhone can take portrait (tall) and landscape (wide) pictures. Hold the phone tall and it takes narrow/tall pictures.