It's just a gas regulating plug that basically has two different sized orifices in it to regulate the gasses that vent from the barrel to the gas block. One setting is for non suppressed operation and the other setting is for suppressed operation.

They have to do this because the pressures inside the barrel will change when a suppressor is installed on the end of the barrel. When a suppressor is installed on the gun the pressure inside the barrel will maintain a higher PSI for a longer period of time that it would if the gasses just blew out the end of the barrel unobstructed.

Do you need it? Only if you are going to fire the rifle suppressed.

BTW, you aren't going to build that rifle. If you want that rifle then you're gonna have to buy it complete from Barrett or a Barrett dealer.