From your subject line, I'm not sure of the exact form or application of magnetic coupling, but it is often used to separate sensitive parts of a power transmission system from a harsh environment. Conversely, it may be used to protect the environment from the power transmission system. In a high vacuum system, for example, outgassing of lubricants and virtual links from blind holes and other volumes can be a problem. Space may also be an issue: minimizing the volume of an enclosure may be a functional requirement. Magnetic coupling could be used to remove components that would otherwise need to be in the enclosure.
