It’s common knowledge that Professor John Danaher, when promoting a student to the new belt rank, also gifts him/her a knife.

For example, Nathalia Santoro was recently promoted to brown belt – and was gifted a blade:

 

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But why is Professor Danaher “fascinated” with knives? He gave a thorough explanation in a recent Lex Fridman podcast episode:

My fascination with knives comes in the sense that they convey a spirit to my students; where a knife is made out of steel and steel begins its ore in the ground.
It’s an ugly, unfinished product which – through the enactment of knowledge, time and discipline – can be transformed into beautiful, shiny steel.

It begins as something that has no real function, and then becomes one of the most important, functional tools in all of human history… Without which human civilization could’ve never even begun.
It’s what separated the humans and took us from the bottom of the food chain and begun our gradual rise to the top of the food chain.

So, it has this immense historical and cultural value.

For Danaher, it all ties in to the metaphorical value of the knives:

But it [also] has this metaphorical value, where a martial artist begins as a white belt; as iron ore. But, over time, transforms into some beautiful, shiny steel which can have immense value.

In addition, there’s a sense of maintenance. As remarkable as steel is, it has a need for constant maintenance. Neglect will destroy the blade, both in the terms of rust and the edge falling apart.
Just as a martial artist; it’s not good [for him] to just learn the techniques – you need to maintain them over time.