The clutch pedal engages and disengages the engine and the gearbox. When depressed, your powertrain is disengaged, causing the same effect as shifting to neutral.
To move off, you release the clutch pedal to set the vehicle in motion whereas when you approach a stop, you depress the clutch pedal to separate the engine from the wheels to prevent it from stalling.
If you depress the clutch pedal while driving at high speeds your vehicle will coast for a relatively long distance on a flat road, gain momentum going downhill, and slow down considerably going uphill.
On the other hand when parking, if you’re on a flat road, the vehicle stops almost immediately when you depress the clutch pedal.