Some punctures are big enough to keep leaking but small enough to seal with some help.
When I was deciding whether to go tubeless I surfed around forums for stories of people having failures. Most of them were either dried out sealant, or rim and tire damage so severe that a tube would not have helped either. There was a persistent problem earlier on of pinch flats and burps, but most tires now have armored sidewalls and/or some people run Cush Core or similar products, especially for DH or enduro. I decided for my riding, which is mostly short hauls in well trafficked areas, I'd just carry sealant. I have a tube now but Darts are on my Christmas list and might replace it. If I were going to Downieville or Moab or something I'd carry more repair stuff.
See for example this (promotional and likely prejudicial) demo by Worldwide Cyclery of the Stan's Dart: