Gw is going to make new line of paints:LINK TO A TOPIC ON A WEBSITE CALLED THE LAST ALLIANCE
(Posted directly from the topic in the link:)
The purpose of them is that they are used as a basecoat with only 1 coat of paint required (even over a black undercoat).
Presumably they are darker as you're shading is then done & you can apply the standard GW paints over the top
black undercoat -> adeptus battlegrey (dark grey) -> codex grey -> fortress grey