The Northwest Passage was the direct water route through the Americas to Asia. This route was highly sought after by explorers in order to increase trade between countries in Europe and countries in Asia.
After Marco Polo returned from China and introduced Europe to the spices, silks, and perfumes people wanted these items and more. The Silk Road was a dangerous long route so a water route would be a good thing. The person or country who found a passage to Asia would gain in wealth.