Why The United States Bought Alaska From Russia

When The United States bought Alaska from Russia in 1867 for just $7.2 Million there were a lot of Americans who made fun of the purchase and mocking names were made up such as "Seward's Folly" and "Walrussia". The piece of land was, according to them, worthless and unnecessary.
However, there was also a group of Americans who recognized the potential of Alaska with their valuable resources like gold and petroleum. In the end, this group may yet be proven right. What happened around the purchase? Why is it remembered incorrectly?