HUGE! A clutch simply provides a disconnect between the engine and axle. It allows slippage at idle and under acceleration. With NO clutching action, the engine could only run with the kart moving. Your gear ratio is fixed. When the engine gets to peak RPM, that's as fast as you're going to go. There is a large sacrifice between speed and acceleration when you only have one effective gear ratio.
A CVT (Continuously Variable Transmission) provides clutching action, PLUS- It is a variable ratio transmission. You are able to take better advantage of the engine's available power, as you have an infinite ratio within the CVT's ratio range. You get a lot more torque off the line, with the potential for greater top speed- even in a kart that is identical to one equipped with a clutch.
CVT's shine on fun, yard, and offroad karts, especially with larger tires. Such a kart equipped with a centrifugal clutch would need to be geared quite low to have any kind of torque in order to avoid burning out the clutch. The liability is that having to gear so low results in a rather low top speed. Having a CVT negates that, providing plenty of low end at a potentially higher gear ratio, while still allowing good top speed.