The answer to your question is:
A. Have the key in your hand.
B. to help put ideas in order of importance
Im guessing you mean where coding is used?
pretty much anything with lights or a button
but, examples:
(some) watches
(newer) cars
planes
boats
credit cards (using one requires code to be ran)
Cash registers
(some) safes
Alarm Clocks
Routers
ATMs
eBooks
Cellular Sattelites
Weather Sattelites
Energy Grids
Water Plants
Sewage
Anywhere where mail is sorted