The human foot is a wonder of engineering and unlike that any other animal on this planet, including those of other primates. However, as well designed as feet are for bipedal walking, they are far less well suited for moving quickly through the water. Fins are a great and affordable way for diving and snorkeling enthusiasts to remedy what nature has left lacking.
There are multiple advantages to swimming with fins, from both an athletic perspective and an enjoyment one. Fins can help divers push through the water much more quickly than they could using only their own feet, especially novice swimmers. The flat, wide, flexible surface of the fins aids in propelling swimmers through the water, giving them a burst of speed they could not normally attain. This is likely why so many residents of the aquatic world have their own natural versions of these useful appendages. Aside from being practical, moving more quickly is also fun and adds enjoyment to the diving or snorkeling experience on your Key West vacation.
Fins also help train swimmers into better long term habits. A swimmer's legs are used more, resulting in stronger muscles. Ankles slowly become more flexible through the repetitive kicking motion needed for movement. Swimmers wearing fins often naturally hold themselves in a more horizontal position, helping them to cut through the water more cleanly. These habits translate to becoming a better swimmer even when not wearing the fins, as well as providing a better workout.
The addition of fins can also free up hands and arms while swimming. When not wearing them, a swimmer naturally uses their hands to aid in stroking through water more quickly, as feet alone simply don't do the job very well. While arms may of course still be used in conjunction with fins, they become less necessary, and swimmers can instead focus on underwater photography, touching the native sea life, or simply holding hands with another swimmer. This is one major reason why recreational snorkelers may prefer to wear them.
Lastly, fins also offer a layer of protection against bites, stings, or icy water. While a regular pair of shoes would seriously hamper a swimmer, even those designed for underwater use, and bare feet could leave them vulnerable to many irritants or dangers, fins shield the foot from potential injury while simultaneously improving function. It's a win-win situation that makes the use of fins an easy choice for most underwater swimmers.