Pitch shifters change the pitch of an original audio signal, usually by up to an octave in either direction.
Short pitch shifts can be used to fatten up a sound. This is done by add the untreated sound to two pitch-shifted versions that are a few cent apart. Large pitch shifts on the other hand, can be used to create parallel harmonies.
Another use for pitch shifting is to correct out of tune notes.
Synth with two oscillators pitch shifted a few cent apart