Many people might not have heard of this before, but it's the concept of making sure that the enemy is as much if not more of a threat than the main character (mc) is to the enemy. Let me give you a non-scifi example. John McClain (from Die Hard) is up against a tower full of terrorist, fully armed with automatic weapons. He on the other hand is armed with wit, and a pistol, to start. If it had only been three terrorist, would that have been as exciting? Maybe.
So back to the curve, if you make a character too strong, then everything gets out of proportion, which can lead to an unrealistic fantasy. I know, unrealistic fantasy, that is what it's supposted to be right, well only if it is explained correctly. If there really isn't this gradual rise or time of learning it feels disjointed and takes you out of the fantasy, hence unrealistic.
No matter the medium, if there is story telling there is a curve, no matter how small. Take a look at your favorite stories and see if you can pick them out.