The program was aimed at preventing US intervention in the war, and required the buyers to send their own ships to US ports, avoiding the threat of US ships being sunk. The program also required all payments in cash currency, rather than on credit - this prevented US businesses from being invested in the success of any belligerent. Because of the conclusion of the Nye Committee, many Americans believed that investment in a belligerent would eventually lead to American participation in war.
Despite its success, this policy soon left European allies (primarily Britain) bankrupt and this forced US leaders to revise the plan. The revised plan was known as the Lend-Lease program, in which the European allies no longer had to pay cash or arrange transportation. Instead, the United States would expect payment at a later time.
In keeping with the Monroe Doctrine, the US didn't actively participate in the war until both Japan and Germany declared war on the US, after which they switched from allied assistance to active engagement.