because depending on your mileage on your car, the locks get worn down. So lets say you have 120k miles, dont have keyless entry. you use the key to open the door and or lock the door everytime. you also use it to start the car. the tumblers in these lock cylinders get worn down to the point where other nissan keys will work. My key opens and starts my friends 240sx. Then he replaced his whole lock system to be safe that someone else could do the same thing and steal his car. this has worked with an older BMW with a diff BMW key and an older toyota as well. all attributed to worn lock cylinders.
-Josh
"Not everything that can be counted counts, and not everything that counts can be counted. " - Albert Einstein
