Sabenaboy.
I believe the ICAO documentation says that a FPL is only valid for the period ETOT + 30 minutes.
When you file a FPL it is checked against route closures, danger areas, military activity, conditional availabilty and so on for the period of vailidity.
If you depart outside the validity of the FPL you could find yourself arriving at an airway that is closed.
The Spanish rule would appear to be enforcing observation of the FPL validity. I would be intersted to know if they have a similar rule forbidding departure at ETOT +30.
It seems to be common sense really.