For the vast majority of heroes, the superpowers they have define them. Doctor Strange's life revolves around being the Sorcerer Supreme, Professor X is defined by his struggles with his telepathic powers, Thor becomes the Odinson when he is deemed unworthy to wield Mjolnir - the list is endless.
It's made clear time and time again that most heroes would be totally different people without their powers, which is exactly why storylines revolving around heroes having their special abilities stolen are so engaging, especially when they are stolen to then be used by a villain.
There's something about seeing folks like Doctor Strange or Superman dealing with the loss of their powers that really makes you connect to them, as you get to see them still fight for good without any of their frills or cool eye laser beams.
On a similar note, these storylines are also interesting for seeing how the various villains involved use the powers that they've stolen - as there's nothing more surprising than seeing Doctor Doom with Daredevil's powers, or Red Skull with Professor X's telepathy. Combine together de-powered heroes, and the villains who've taken their powers, and it's scientifically impossible to not have some comic gold on your hands.