If you know all there is to know about something then you have complete and total control over it. A mage with Entropy 2 knows everything about randomness and weaknesses, and can thus control them. This important, and tied to Arete. This is the main difference between mystics and mages. Mages generally turn outwards, learning how to control the world and seeking to understand it.
Mystics turn inwards, and regard external pursuits as a waste of time.
Example is Ursula Le Guin's Wizard of Earthsea, traditional demonology etc. This is the main tenet of classic hermetic magick, and much of shamanism. If a mage knows the true name of something, it will be in his control. Finding true names is of course quite hard, and different traditions have different methods. Dreamspeakes make journeys into the Umbra to seek out the spirits and ask them to reveal the name.