Single bit DSD based DACs have more random noise in their outputs than other DAC designs. That noise isn’t correlated to the material being played so it ignored very well by our ear/brain. That noise is low enough to typically be inaudible at realistic listening levels but high enough to be audible with the gain that some systems have when music isn’t playing.
The problem comes with very high gain (or very efficient speaker) systems. If the typical volume setting of the DS isn’t above say 60 or so then the analog noise floor of the DS can be audible at those levels. As mentioned above to address this we provide the 20dB attenuator, when engaged it lowers the noise floor by 20dB and allows using a volume setting that’s 40 ticks higher.
It is the case that many of us can hear the noise if we listen close to our speakers when the music is paused, but by either using an external preamp or by setting the 20dB attenuator to the “best” setting for a given system the noise shouldn’t be audible when playing at typical levels.