Some examples:User needs a solution which will need adjustment 'in the field'. When adding a wifi-shield, he can send Serial Data to the Arduino to adjust the program.User needs to write a large program and using external memory and an interpreter is able to (at the expense of speed.)User builds program in the Arduino IDE and inserts a library to the interpreter. After uploading he sees that it does not work as expected and uses the Serial Monitor / Interpreter to probe the Arduino for debug info.
Best of luck implementing JIT on an Arduino.