What is it – do you need it?
Not too technical and using some generalizations, here is an explanation of re-flashing, as it relates to getting your car keys made.
Basically, very basically, some of the early transponder key equipped vehicles required you to have at least one working programmed master key to open the programming sequence which would add or program additional transponder keys for your car. If you lose all your keys to one of these vehicles there is no way to program more keys because you don’t have a way to tell the cars computer that you want to do this. You could have new keys made, but you could not program them, so the car would not start.