When making simple hand tools, a ferrule is an important addition.
The ferrule on the knife was made by heating a nut on punch, to a nice cherry red an then beating it with a hammer. This gave it a good angled inner surface that jams on tight to a wooden mortise. This makes for a stronger tool.
A ring at the back end of a tool can keep it from being destroyed by the process of wacking the tool with a mallet. Ferrules are important, and there are a lot of ways to make them.