A knife is a universal tool, of endless uses for camping, fishing, foraging, outdoor cooking, do-it-yourself repairs and outdoor life.
While many travellers have reasons to use a knife, many countries and venues regulate carrying of blades.
While travelling in populated areas, the knife is mostly a convenience, allowing you to make slices of your bread etc. But every hunter or fisherman knows the importance of a good knife, and for the wilderness the knife is the one essential tool: with a good knife an expert can make fire, lodging, transport and anything else needed.
Though knives are made and used all around the world, some places are famous for knife-making.
In many countries, unauthorized knife-carrying is criminalized. Even in countries where knife-carrying is not a crime, some restaurants and other venues prohibit knives. Venues in high-crime areas, as well as vulnerable institutions (banks, nightclubs, embassies, etc) might require guests to pass through a metal detector.
In some places a pocket knife with a blade shorter than a designated length (the palm of your hand is usually as long as it gets) may be allowed, where other knives are not.
When flying pack any knives into your check-in baggage as carrying a knife into the cabin is strictly prohibited.
Officially, knives of any kind are prohibited on trains in China, but in practice packing a very small pocket knife for peeling fruit etc usually is not an issue.
Carrying a blade for self-defense is not recommended. Just like other weapons, a knife is more likely to raise the stakes and cause more damage than it prevents.