My understanding is that it is a Clubmark Accreditation issue. Which is a nonsense reason really, I neither know nor care what clubmark does and it wouldnt affect my personal enjoyment of the sport if we lost accreditation. Someone else told me it was Insurance, and yet another person that it was BOF. Maybe it was the government, who cares at the end of the day?
Whatever the reason, the fact is that giving or registering emergency contact details somewhere will likely be one less thing for a stressed organiser to worry about if something were to happen... I was very impressed to notice that for the Urban Summer series the Fabian entry had a way of storing contact details.... seems a pretty simple way of doing it.