Names are very important in the faerie realm in regards to power. It is a common belief that if you know the true name of something you have power over it.
This belief is not only found in faerie culture either. In some cultures people believe that your middle name is your true name and you have to keep it hidden or the devil will discover it and try to steal your soul. In North American households, a child knows when they are really in trouble because their parent will call out their entire name, first, middle and last, and when that happens I’m sure every child has experienced that electric shock shoot up their spine. Or how you snap out of a daze when someone calls your name.
There is also the common belief that when you put a name to something you’re afraid of you stop being afraid of it, when you come to terms with it in your mind and stop rationalizing or running from it. The is illustrated in the children’s series Harry Potter. In the novels, there is a very evil wizard named Lord Voldemort and everyone is afraid to say his name, which seems to make them more afraid him. By not being able to say the name, this wizard is out of their power, more powerful than they are and only Harry Potter (the protagonist), a special group of people called the Order of the Pheonix, and his school principle Dumbledore (the most powerful wizard of all time) actually call Voldemort by his name, and it is these people who stand a chance at defeating him because they are able to say his name.
Back to faeries now :p So knowing the true name of one of the fae gives you complete control over it which is why they will go to great lengths to keep their true name a secret. Keeping names hidden not only protect faeries from being controlled, it also protects mortals from being harmed by faeries. An example would be the well known folk tale My Own Self, which you can read here.