Opaque Grays are Black and White ink mixed together. You can do this yourself, but having then pre-dispersed into four target values in large batched makes them work better and allows them to be more consistent.
All Grays can be mixed in to neutralize or mute other color. This can sometimes make it easier (rather than using complementary colors) to maintain (or target) the desired value outcome and effect. In other words, mix a little Gray into any part of the composition you do not want stand out. This can be a bit easier than using primaries and chromatic mixing.
Mixing Opaque Grays into color can increase the longevity of a color. This is because the Opaques contain Black ink which typically lasts longer in the skin than color.
Opaque Grays can also be packed in like color to cover up old tattoos. It can look great alone, mixed in with full color, or even looks great side by side with gray washes. The Opaques will simultaneously lighten and even out the dark pigment in the skin, but only a nominal amount per session.