That isn't the reason for FP.The reason the Fastpass system was created in the first place was to make the parks more enjoyable for the guests because before it was in place you literally had to wait two to three hours just to get on one of the more popular attractions everyday.
FP was created to increase guest spending in the parks. Strategic Planning realized that when standing in line, guests were not able to shop and eat in restaurants. FP was put in place to free up time in lines to allow for a greater spend. It worked, and guest spending has largely been on the increase ever since.