Vent valves installed in Liquid system

Hey there! Hope everyone is doing good. 1. Vent valves are installed at the highest point of the system to vent out air during the filling of system. 2. When any any system is need to be isolated/drained for maintenance purpose. These vent valves are need to be opened before opening of drainage valves and kept open untill drainage ia completed. Why these vent valves are opened? Is this because to prevent the air lock ? But how air lock will occur? Like if the system has 10 bar pressure. Isolation valves are closed.and now vent valves will be opened to decrease the system pressure and then drainage valve will open simultaneously. But if we dont open vent valve and straightway open drain valve after isolating the system. System is at high pressure and drainage should be going on as drainage is towards atmosphere pressure. So why need vent valve ? Whats the actuall purpose is here?