You are supposed to lift off the accelerator as well when engaging clutch.
Also when engaging clutch you are lifting the 'load' on the engine it will
therefore run faster.  I have an auto and when at a standstill changing from
drive to neutral increases revs.  This too is because the 'load' has been
removed from the engine.  Basically the engine is no longer connected to the
transmission once the clutch is fully engaged or the clutch has so little
contact with the drive plate it makes no difference to the load.
Unfortunately there is a slight contact between clutch plate and drive plate
or flywheel.  Therefore when at a standstill using the clutch instead of
placing in neutral will increase wear of the clutch plate.