Knives are expressly designed to cut. They serve no Purpose (ignoring cooking, as those are basically a proxy for proving how well one can cut with a knife). There is no reason anybody outside of the kitchen needs a knife to carry out their day to day jobs.

Encryption is expressly designed to hide terrorists. It serves no purpose (ignoring privacy, as that is basically a proxy for proving how well one can hide from the government). There is no reason anybody outside of the military needs encryption to carry out their day to day jobs.