If you look at the Envelope Generator, the voltage goes up (attack) as a key is being pressed on the synthesizer keyboard or the sound starts, goes down a little (decay), stays level (sustain) and goes down again (release) as the key is being released or the sound ends. So it's quite natural to have the envelope modulate the amplifier (VCA).
Above is what you see when you go into SYNTH EDIT for synth1 or synth2. In this post, we focus on the VCA (Voltage Controlled Amplifier).
The slider switch enables you to choose the modulation source signal: Envelope Generator (EG) or GATE. As you probably know, the gate lets the sound pass through, between the pressing of a key and its release. Voltage is 0 when the gate is closed and at the maximum when the gate is open. So clearly, it can be used to modulate the Voltage Controlled Amplifier (VCA) although it kinda defeats the point of having an envelope generator at your disposal. Having the envelope modulate the VCA makes a lot more sense.
The LEVEL knob adjusts the level of amplification to be given to the signal. It's more or less a volume knob if you will.
The DRIVE knob on a synthesizer is usually there to boost a low signal prior to distorting (overdriving) it. On the Nintendo KORG DS-10, it's not very clear what it does especially since the actual KORG MS-10 doesn't have one. Best guess is that it is simply an OVERDRIVE knob to produce distortion.