Without a vocoder, filtering a synth with a fixed EQ sounds static. The vocoder’s changes the synth’s spectrum up to hundreds of times per second, tracking your voice’s formants. The result is temporal coherence : the synth follows your pitch contour and timing, but retains its own harmonic richness.

More bands result in clearer, more articulate speech. Fewer bands create a more "lo-fi" or robotic effect.