The purpose of the JL Clean Sweep is to cancel out any factory EQ settings the manufacturers have preset into the factory stereo to compensate for the factory speakers and acoustics. It's job is to send a "clean" signal to your external processors and amps.
It's basically a highly advanced line level convertor. You run your speaker wire into it, then install the calibration CD into the headunit,and set the voume at 75 % play the CD and about 20 seconds the JL cleansweep's internal equalizer will adjust the factory preset eq settings. Then send a clean signal through RCA's.
For a regular system this is definitely not needed. For a very high end system you would benefit from this.
Unless you are doing a very high end install, just get one of these
NE-774V
The only feature the JL Cleansweep adds that an aftermarket deck would give you is auxilary inputs.
Another thing to note is that you are supposed to use the Cleansweeps independant Volume control, and not the factory headunits.