Pretty much internal combustion engine driven vehicle needs a clutch of some sort. A clutch allows the engine to stay running while the vehicle is still, slip and engage the driveline, and prevents excessive load on the engine.
On stick-shift cars, the clutch is manually operated.
On most go karts, the clutch automatically responds to the centrifugal force created by higher engine rpm. At idle, the engine is disconnected from the driveline. When rpm picks up, centrifugal force pushes out weights inside the clutch to engage the driveline.