Both of the CS-3's main functions, Compression and Sustain, serve to tame very loud piercing notes/tones while raising up low level notes you play, as well as sustaining the tone. If there is noise in the signal to begin with from a high gain setting for instance, that noise would potentially be accentuated by the CS-3.
We recommend experimenting with different setups until you get the sound that you’re looking for.
Here’s a couple links to an article on the Roland/BOSS blog site that should give you some ideas: