You clutch in to prevent the engine from stalling.
At 60 kmph, when you step on the brake, you slow down. If you do
not clutch in, you will begin to feel the engine knocking.
Clutch-in to prevent further knocking and stalling. With practice
you can automatically clutch in and shift to a lower gear as you
slow down, and have the engine running smoothly. This skill is
useful in a traffic jam.