Sometimes swimmers wear fins just so they may maintain the same speed as other swimmers working out with them.
Other times, a swimmer might want to move to a faster lane.
If your only purpose is to get into better shape, and you don't want to compete, then using fins is fine.
There are some fins (like Zoomers) that will actually help you to burn more calories than swimming without.
For those involved in triathlon racing, though, there is more to be concerned with than just burning fat.
Having said that, there are some reasons why you might want to use fins.
One situation would be to build leg strength.
Another reason is that you'll be able to use more oxygen throughout your workout.
This will allow you to support more muscle mass use, at a higher intensity.
It is important, though, to realize that using fins regularly is a mistake, especially when the fins are intended to keep the swimmer from sinking in the water, or to make him or her a faster swimmer.
People who will particularly benefit from training with fins is runners.
Often, one who is not accustomed to swimming will often produce little to no kick, or may even move backwards when they kick.
Using fins for a short period of time can improve stroke and speed.
For this purpose, Zoomers, or any of the shorter variety will work.
The reason this works is because fins allow you to concentrate on your upper body movement, and your endurance as a whole, rather than having to constantly be conscious of your leg movement.
When you consider that about ninety percent of your propulsion through water comes from your upper body, it makes a lot of sense to fine tune this section before moving on.
Once you have the upper body movement down, you can start to slowly move away from using fins.
You'll probably notice the difference from your "pre-fin" days, even in your legs, because of improved ankle flexibility.
Zoomers can be a great tool for triathlon training.
But remember that they can become addictive, and may impede your natural improvement in stroke and times.
If you use fins correctly, without abuse, they can add huge benefits to your swim and overall freestyle stroke.