The Press & Lock mode is designed for long exposures. If you do not want to keep your finger on the screen all the time, you can use this mode. The shutter will open with a touch on the button and it will stay open until the next touch.
When you touch the button, a counter will start at the bottom of the screen. The camera must be in BULB mode to have flexible exposure.
You can reach to Camera Control screen from this screen by using camera icon located at the top left. It is also possible to see the taken photos using Preview button located at lower left corner.