A device that allows seamless gear upshifts and downshifts without throttling off or using clutch is known as a quick shifter, commonly used in motorbikes. The elimination of setting clutch and throttle before and after gear shifting allows rider to shift quickly. Basically this tech was founded for the desire of faster lap time in racing as it quickly shifts gear (less than 50 millisecond) and enhances the performance of motorcycle.
It is completely not necessary for the street but there is no harm in using it.
Generally we pull off the throttle, use clutch to disengage the gears from transmission, shift the gear and back to acceleration. But in quickshifter there is a sensor integrated into the shift linkage that senses the gear shift action and sends it to the bike’s ECU (Electronic Control Unit) that momentarily stops ignition allowing reduction of load at the transmission and in that millisecond pause in power it disengages the gear, making a new gear to engage into its place without any requirement of clutch or throttle adjustments. It takes milliseconds to shift and those half seconds adds up when we are racing. The only drawback of using quickshifter regularly is that your skill of clutch and throttle tuning is likely to get nasty.
With quickshifter, gear shifts so fast that driving becomes uninterrupted and many bike manufacturers are giving out quickshifters like Suzuki GSX-R1000 and touring bikes like BMW K1600.