Indeed but it is a helluva lot easier to file the flight plan from one's hotel room the night before; no hanging around at the airport then
This business of using national websites for out-of-country filing (e.g. using Olivia for a flight wholly outside of France) seems to vary. I have heard it works, and I have also heard that the flight plans just vanish. This is obviously no good because one isn't going to discover it has vanished until one starts up. Whereas Homebriefing.com officially works worldwide.