Play Lock saves battery by turning screen off while game is running in background. When you need to have the game opened, and it does some automated job (like farming in Ragnarok M) or you need to walk around in augumented reality games (like Pokemon GO), then you can turn the screen off and save battery from phone's screen usage. It really works!
Play Lock saves at least 2 times more battery than built-in Pokemon GO battery saver! Why? Because Play Lock really turns the screen off and Pokemon GO just shows black screen, and even on AMOLED screens it's the same - read the science on XDA-developers site
Or in short "Displays also consume power just by being turned on — the display driver becomes active and ready to send and receive signals from the SoC to the thin-film transistors."