First of all, to answer your question, let’s say who William Seward was. In early years he was a Governer of new York and the united states senator . from 1861-1869 he was United states secretary of states. He had a major role in purchasing Alaska. Seward and lots of different Americans believed that Asia would become a crucial marketplace for the country's merchandise, and expected that American state would function a base for American merchant ships. this is the main reason why Seward wanted United states to purchase Alaska