Why Are Trumpets Important In A Fanfare?

Because they were the original fanfare instrument. The purpose of a fanfare was to precede an important announcement, such as the presence of nobility into a household. This was best done by the loudest musical instrument which, at the time fanfares were used, was the trumpet. Some of the richer families had as many as 16 trumpets sounding the call at their presence. 16 violins would not have had the same affect.