Swimming with any equipment is a good way to train in swimming. however the use of fins are just as good for training as without using them. Just like at a gym, one can use various tools for training to gain muscle or lose weight.
With fins, each has a special function in which they help the swimmer to attain body strength. There are many fins that each have a special advantage to training a swimmer.
They can also relieve stress on shoulder joints—something most swimmers experience from time to time. Training with fins also helps build muscle while improving your up-kick, ankle flexibility, overall body positioning, and conditioning
Wearing fins improves your body position – it is easier to keep your body higher up when wearing fins, as your kick becomes more efficient due to the larger surface area so therefore you are higher in the water.
This is advantageous for swimming because it can help you when focusing on other aspects such as drills, which are much easier when wearing fins.
In addition, when you take your fins off you become aware of how hard you have to kick to keep your body higher up in the water, and having your body higher up in the water makes you faster as you have less frontal resistance.