"A weapon or tool doesn't have to have its one designated use, it has versatility." - Naked Snake (Big Boss) & Solid Snake (Paraphrased by me personally because...I don't remember the exact line.)

The Kunai knife we know today was actually a farmer's tool for digging holes in the ground.

The reason it became a weapon is because Ninja or Shinobi (They're actually the same name designation for the same group) are taught to use everything around them in the field. That's why we see the famous table kick that is used to distract, occupy, or even hit the opponent so that the one who tossed the table can have time to get ready to either prepare an attack or escape.

Right.