I like the fact that knives represent the combination of so many different factors into a single final product. Design, aesthetics, engineering, art, metalworking, woodworking, chemistry, science, beauty, functionality, art, and more.
Knives are also such a critical part of our lives, whether we personally handle a knife on a daily basis or not. Almost all the food we eat is likely to have come in contact with some kind of knife at some point in its journey.
I would argue that knives are one of the most practical and necessary tools for human function. We could probably get by without them, but I think we would be surprised at how much it would impact our lives.
You can find an interesting read by searching for Forbes, No. 1 The Knife, on this topic.