Bypass is essential for evaluating any audio processor: compression, EQ, saturation, or reverb. Without bypass, volume differences between processed and unprocessed signals fool the ear — louder always sounds 'better.' Proper bypass maintains identical levels so you judge the actual effect.
Most plugins and hardware include a bypass button. For critical listening, use level-matched bypass (the processed and unprocessed signals are at the same perceived volume). Many engineers bypass plugins regularly during mixing to ensure they are improving, not just changing, the sound.