As of March 2015, the United States Postal Service (USPS) sets a 1-ounce weight limit for a First-Class letter with a single stamp on it, so the second-ounce rate is in place so that people can mail letters of up to 2 ounces by including a special additional stamp for a lower cost than using two regular stamps. First-Class Mail is a service to send light letters and packages between 1 and 3 business days throughout the United States.
The USPS does offer a benefit to businesses where they can send certain cards and letters without having to pay for the second ounce. Instead, they only pay the 1-ounce postage rate. However, this benefit is not for sending a single letter but is for sending presorted and automated mail.