MIDI Compressor is a utility device that allows you to make non-linear changes to the velocity of MIDI notes.
You can use MIDI Compressor to increase or decrease the dynamic range of a performance, which makes the playing sound more (or less) expressive. Since MIDI Compressor allows you to automate and/or Remote(TM) its velocity mapping, you can also adjust the dynamic range of performances on the fly.
Another great use of MIDI Compressor is to tweak the velocity response of an Instrument device to better match your master keyboard and/or play style. For example, if you feel that an Instrument outputs loud notes when you play softly you can add a MIDI Compressor above the instrument and tweak the response to become more even. Or if you use multiple master keyboards, you can create MIDI Compressor patches for them so that your playing sound similar regardless of which keyboard you use.
And, of course, if you simply want to adjust the velocity of a performance, just add a MIDI Compressor above the Instrument device and turn the appropriate knob. Much quicker than selecting note clips in the sequencer and entering values in the tool window!