First, please read these guidelines on using any training device.
Fins can be helpful in some situation.
You want to avoid creating new problems while trying to work on some part of your stroke. But, if you can use them sparingly, fins can permit you to generate some velocity and add a slight amount of lift to your legs while concentrating on some part of your upper body. They can be particularly helpful when working on Skate Position, or Slow-Motion Strokes (such as Swing Skate or Swing Switch drills).
Or maybe you want to do drills in a lane that you are sharing with other swimmers and need to keep moving down the lane to avoid blocking anyone else. Fins can push you along while you do slow, upper-body work.
If possible, use short fins which allow you to keep the feet closer together and move them more like you would when kicking without fins on.
If you are looking for a recommendation, we like to use the Finis Postive Drive fins.