A tubeless tire with sealant can often close a small puncture on its own. The wheel needs to turn, or the damaged spot needs to face the ground, so the liquid reaches the hole. Not every loss of air is a small tread puncture, though: leaks can also occur around the valve or rim.
For a larger cut or pinch damage, sealant may fail to hold air. Schwalbe recommends a spare inner tube in that situation. Fitting it means removing the tubeless valve and wiping away as much liquid as possible from inside the tire. Before riding on, check that the damaged tire can still safely hold the tube and air.
The distinction is between a small hole and damage to the tire body. Sealant reduces interruptions, but it does not replace a way to deal with a more serious failure. A spare tube and pump therefore remain sensible equipment for a longer ride.