What diffetence does it make to a car when the fuel injection system is reprogrammed?

Everything changes somewhat as the car ages, so the new car computer parameters that were originally programmed into the computer may need updating. Also, the factory learns things about the engineering design in the mean time that result in Technical Service Bulletins being issued for revised parts and adjustments etc. Since your Renault dealer did the work, we'll have to assume you just need to have them follow up one more time on the work they did. Give them another chance to hook up their OBD-II scanner and recheck that your engine is operating normally. Tell them your current complaint. Is it still cutting out at low speeds? Is the engine idling rough and running rough with reduced power?