Final answer:
The purchase of Alaska brought significant benefits in terms of resource acquisition and strategic geopolitical positioning for the United States, thereby justifying what was initially criticized as 'Seward's Folly.'
Step-by-step explanation:
The benefit of purchasing Alaska according to US supporters was primarily the addition of a vast territory with rich mineral resources to the country. The purchase was initially criticized and referred to as "Seward's Folly."
However, the Alaska Purchase granted the United States access to an area rich in natural resources, such as fish, furs, and especially gold, which was discovered later, sparking the Klondike Gold Rush.
Moreover, acquiring Alaska was a strategic move that expanded the United States' presence in the Pacific and competed with European powers for economic and security interests.