The ECU (Engine Control Module) is a digital component embedded in latest vehicles that holds, reads and control things like key data, CO2 sensor output, engine RPMs, engine performance, fuel to air ratio and more. 

The ECU is a EEPROM (Electronically Erasable Programmable Read Only Memory) computer chip which means the chip retains its data without periodic electronic power; loss of electric power will not cause the chip to lose its programming data.  

Some first generation transponder systems weren't designed to allow for programming new keys via the ODB2 port so on some vehicles there is no way of making a new key without reflashing or replacing the ECU. Also, if the ignition or the ECU is being replaced on a vehicle a reflash service would also be necessary even if a key already exists.  

Reflashing the ECU involves deleting the read only program and uploading a new one to the vehicle. 

Here is a list of vehicles that require ECU reflashing to make/add a new key: (Not all vehicles require an ECU reflash if simply adding an extra key to the vehicle memory).