It doesn't change the sound of the mp3 at all, just the signal level, which is why it's kinda neat.
But a playback that supports ReplayGain, and R128Gain, is really where it's at. And even with audio processing, it does make a difference because the processing can be relaxed because you know that the input will be within a certain range, and when things get mixed together like two songs or imaging or commercials... nothing will be a lot above or below the levels of the other, so the processing will also sound more stable even when its adjusted to be less aggressive.
