Playback Rate changes the playback rate of the entire sound. The sound plays faster (or slower) and its pitch is higher (or lower), similar to playing a vinyl record faster or slower. Essentially, this just changes the sampling rate value shown in the status bar. To change the playback speed without changing the pitch, use the Time effect.
While the playback rate of the audio device is controlled with the speed fader in the Control window, this has no effect on the file's data. You must use Playback Rate for the change to be permanent. Some file attributes may have a fixed sampling rate. A warning is shown in that case. An MP3 file with "Layer-3, 44100Hz, 192kbps, stereo" attributes, for example, is always saved with a 44100Hz sampling rate. You must use Save As to select attributes with the rate you want.
See Also: Doppler, Pitch, Time, Resample, Speed fader, Presets