Why A transistor used as a switch is driven hard between two extremes: cutoff, where base current is zero so essentially no collector current flows and the device looks like an open switch, and saturation, where enough base current is supplied that the collector-emitter voltage drops to a few tenths of a volt and the device looks like a closed switch. Operating at these endpoints minimizes power dissipation, since either the current or the voltage across the device is nearly zero. Linear operation between the two extremes is deliberately avoided in switching service.
Watch out The active region between cutoff and saturation is where a transistor works as a linear amplifier, not a switch; peak and valley currents describe a tunnel diode, and enhancement and depletion refer to FET types.
Switch = both ends of the curve: fully off (cutoff) or fully on (saturation). The middle is for amplifiers.