You can probably guess what this topic is going to be about!!!! By grouping all your functions and placing them in individual files makes the "re-useable" C code re-usable!!
Lets say you need to talk to and from an external device... Lets say an EEprom! Lets say a 24LC256.... If you have done things similar to me, and built a PCB board that suits the needs of your project / job... Nothing will change in your code that relates to that device and nothing will change with the PCB you have made......