Use of ReplayGain preamp

[theotherme]

Hi. Still using V 7.3.0, very happy with it. 
 
I’ve been experimenting a bit with ReplayGain in MPD config UI. I am not sure I understand the purpose of the ReplayGain preamp parameter. I’ve set ReplayGain to “Track”, which, as expected, seems to be adjusting the volume using the value in the track ReplayGain tag.
 
Then if volume if being adjusted using the Album or Track ReplayGain value, what’s the purpose of the ReplayGain preamp parameter? Info says “The gain (in dB) applied to songs with ReplayGain tags. Range is -15 to 15”. Isn’t is using the value either the Album or Track tag?
 
Thanks!