If you are new to this you might say I have a spare wheel or even two spare wheels, why do I need a plug kit? I can tell you from experience you can definitely have multiple flat tyres in a single incident. This can easily leave you with more flats than you have spares leaving you stuck on the side of the road.
But a simple piece of repair equipment that will fit in your pocket, weighs about 200 grams, and will cost you around $20-30 can get you out of trouble until you get to the next town. The humble tyre plug kit, as the name suggests, repairs