Why was an all water route to Asia important to the Portuguese?

Commerce was the main reason that all the major European nations were seeking a water route to Asia. The spices and other trade items found in places like Japan(Cipango), China (Cathay), the Spice Islands (Indonesia), and India, could be purchased and resold back in Europe but the overland routes were so dangerous, too long, and expensive, that the profits made were not worth the risky investments. If Portugal could find a water route to Asia they could challange the Italian monopoy on Asian trade, they had an established government willing to train sailors and invest in the attempt to discover water routes, and a commercial class willing to invest in such ventures. Plus, the profits reaped would make Portugal the leading maritime power in Europe.