Why did william seward want the united states to purchase alaska?

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

Because he proposed that all the treaties should be made through the league of nations.