[Mithadan]

In Letters, a clear distinction is drawn between Valar/Maiar who merely "clothe" themselves in the bodies of the Children of Iluvatar and those who incarnate themselves in such bodies. When the Valar/Maiar clothe themselves in a body, they may "change their clothes", i.e. take a different image (the Sil describes multiple appearances used by Yavanna, or go unclothed. When incarnated, they are linked to their body as are Elves and Men, and if their body is slain, their power is severely affected at least until they can develop a new body. Morgoth was incarnated in his Dark Lord body and Sauron, after the fall of Numenor, also was. The Istari, as a condition of accepting their mission, were incarnated. Being incarnated, as opposed to clothed, also reduces the abilities/power which they can manifest. The Istari were further limited in their ability to use power. Mr. Underhill, in another thread argues that this additional shackling of their power was voluntary and by agreement rather than a "physical" or actual limitation. Little of this is express or clear from LoTR, though it is implied at times.