Fins can allow you to swim faster -- much faster.  But, to properly and productively swim with fins, you must allows the feet to set the tempo.  That is, in order to achieve the fastest swimming speeds with fins, the legs must be fully engaged, executing a fluid, and continuous cadence.  In order to achieve this fluidity, you must ask the arms to synchronize with the legs.  This is a stark contrast to the way that we typically conceive of arm-leg synchronization, where we try to make the kick mesh with the arm stroke.  So, we first focus on a fluid kick, and then the beauty of the fins can be realized.  

First off, we can use the fins to swim at "supra-maximal" speeds, that is, at a higher velocity that we are capable of swimming without them.  At these foreign speeds, our body gains an insight into how it must alter itself in terms of control, rigidity, and elevation while moving faster.  The body can learn from this experience, and ultimately, nudge itself towards greater speeds when swimming sans fins.  Also, at these high speeds your hands can gain experience with the subtle shifts in position, pattern and angle that are necessary to assist in the process of propulsion.  This too is a valuable peiece of information -- if you are swimming high speed with fins, and feel like your hands are adding nothing to the speed, keep experimenting, trying to feel the water, moving your hands faster or more authoritatively, or do several strokes without breathing in order to focus on head, elbow position and the finish of your stroke and how they alter your ability to propel.