Compressors and limiters work on small time scales (milliseconds), and the distortion that comes with these micro-adjustments is not always desirable.
In some applications, it's better to "manually compress" a wave by drawing a gain envelope on a time scale of seconds (or even minutes!) instead of milliseconds.
This technique is especially useful in recordings with a wide dynamic range, like the piano music I record. It's nice to bring up the volume a bit in the quieter sections, so that the loudest peaks don't limit the overall volume of a piece.
Unfortunately, manually drawing volume curves can be quite tedious: there's a lot of trial and error involved in getting the levels right for each section so that nothing goes over the top and quiet sections are consistent with respect to one another.
Part of the problem is that there's not an obvious visual correspondence between a gain curve you draw and the waveform you're trying to shape: when the waveform goes down, the gain curve goes up — and not necessarily by the same distance on the screen!
Enter Gain Shaper. This audio plug-in will just adjust the volume of your audio — nothing more! — but does it in a way that will take advantage of the on-screen display of the wave.
You draw an automation curve that visually matches the shape of your wave, and Gain Shaper will adjust the volume for you based on the curve you draw.