Re: What is the purpose of the drive or flex plate
I think your talking about the clutch disk. The clutch disk rides on the flywheel and is engaged/disengaged by the throwout bearing and pressure plate when pressing in or releasing the clutch pedal.
If you bolted the transmission directly to the engine then there would be nothing to disengage the transmission when you stopped or to keep it from taking off when you started the engine. In a automatic transmission the torque converter regulates torque from the engine to the transmission. In a manual transmission, the clutch regulates it.