It makes sound standalone. The USB port is to charge, reprogram stuff, and also stream data to your computer (if you want).
Internally, it has all of the Shnth/Fish synth capabilities, so tons to program/edit/change/alter, or you can just use existing patches. Then there’s a couple of onboard modes, so either combining the signals, or an (analog) ring modulator, ala the oldschool Namasitar.