Why do you need a AC to DC adapter? 

The AC to DC adapter transforms the alternate current (AC) input to direct current (DC) output. For this device, it will convert the main electricity (100V-240V AC) to 12 volts (DC) that you need. You can use this adapter to power up most of the application that need 12V input, such as Arduino Uno, Arduino Mega, SK40C and etc. You definitely need one if you are building your own circuit with a 12V input!