Wearing swim fins increases the amount of resistance your muscles experience as you kick, guaranteeing you'll put your leg muscles to full use.
Wearing swim fins actually forces the ankles to fully extend into a straight line, similar to a ballet dancer on point.
Since wearing swim fins increases leg strength and flexibility, the lower half of the body is better able to remain level with the upper body, a much better position for achieving forward thrust.
They are often part of the standard gear used during swim lessons, along with other equipment such as swim fins and swim caps.
While the history of swimming goes back well before Ben Franklin's time, his simple idea for swim fins has revolutionized water sports, and is constantly improved upon today.