Let’s say you want a brighter sound. One way to achieve this is by using a EQ which will push the volume of the high frequencies of the sound. No new frequencies are introduced this way only the balance between frequencies are adjusted in volume.
Another way to get a brighter sound is by using a distortion because a distortion can add extra harmonic content to a sound. It’s the opposite of subtractive synthesis which uses filters to remove harmonic content. A distortion can create a more complex, a more interesting sound. Often a distortion excels at the midrange (both lower and higher mids) instead of the treble range. The midrange is the most important range in my opinion and adding some complex sounding additional harmonic content is the trick that drives not only good old rock & roll but is still the magic ingredient of most popular music.
My favourite distortion “box” at the moment is the Battle-Axe Sound Destroyer, a Rack Extension for Propellerhead Reason. It can add subtle musical sounding even harmonics also well as extremely overdriven sounds.
A simple trick to find out what kind of extra harmonic content the distortion adds to a signal is by using a clean sounding sine wave (Reason’s Thor Synth with Wavetable OSC or Ableton Operator are perfect for this). Here’s the spectrum of Reason’s Thor Synth without any distortion:
And here’s that same tone but feed through that lovely Battle-Axe Sound Destroyer device:
The additional harmonic content is obvious. Something you will never be able to achieve with a EQ, compressor or any other effect. If you use these kinds of effects on a sine wave the sound will stay the same. But with a distortion the sine wave (the most basis waveform there is) changes into a more interesting harmonic sound.