One of the things writers wrangle out is 'how do you challenge someone who is basically invincible'? The easiest way is to hit him in his weak points -- for a long time, the number one thing was kryptonite, so it seemed like the green rocks were as common as muck. The second is magic, which seems to be exaggerated in effect. In theory, he is no more or less resistant to magic than a normal human. In practice, waving a magic wand at Supes tends to make him crumple quickly. Mental powers are the third, and tend to be exaggerated in the same manner as magic... since it's one of the things that does work on him, it seems like it always, unfailingly works to its best possible effect on him.
Basically, it boils down, I think, to the idea that if Superman is not invulnerable to something, it's treated as though it is guaranteed to defeat him.