Increase Speed: swimming with fins propels you through the water faster and your legs work harder then they would without fins to maintain the speed. You are able to maintain that quicker pace wearing swim fins, but without fins you would quickly become fatigued. Using swim fins will ultimately make your leg muscles stronger to enable you to swim faster.
Improve Leg Strength and Flexibility: Fins will force your feet to extend and flex beyond their normal range of motion, plus the extra force on your ankles as you kick will improve your ankle flexibility. The water resistance fins create, because of the increased surface area of your foot, builds up strength in your legs.
Improve Body Position: The buoyancy of fins means that they will help you to keep your legs and overall body position higher in the water.
Improve Racing Technique: Swimming with fins allows you to swim at typical race speeds without the fatigue that that brings. You are therefore able to swim as in a race, which means your body movements are faster than they are in typical swim training.