There are number of benefits from using fins.
Firstly, an increase in the flex of you ankles which in time will mean a more effective swim position, as there is more resistance placed on you ankles.
Secondly, they are also very good to use when you practising body position, upper body and breathing drills as you will travel at a great speed than you would normal (without fins) and therefore you tend 'float' better due to increase in speed (less drag) allowing you to concentrate on what you are trying to develop rather than actually staying on 'top' of the water as you do when swimming normally.
I'm sure there are others but off the top of my head these are the one that sprang to mind.
There you go - clear as mud, defo worth the use they can really help in developing your swimming technique.