Why was the movable type of the printing press so important for publishing?

Because it allowed the mass production of books and allowed a previously unknown level of information to be disseminated.

Explanation:

With letters and embossed symbols carved in metal. The type of mobile printing, invented by Johannes Gensfleish, known as Gutenberg, allowed the mass printing of books - which were once handwritten - beginning a revolution in Europe in the mid 1455s.

The technique was innovative, but it was not the pioneer. Since the 7th century, calendars and holy books have been printed by the Chinese - who used about 400,000 wooden carved ideograms.  But Gutenberg created tougher moving types that could be reused in other print jobs. Thus, books are no longer exclusive to nobles and clergy. By 1489, there were presses like his in Italy, France, Spain, Holland, England, and Denmark. By 1500, some 15 million books had been printed.