Black is a negative colour. It is maximum darkness, but complete blackness is rare, because there is always light. White on the other hand is the opposite: white is maximum lightness and in theory reflects all light, but even the whitest white still absorbs a small amount of light. Grey is every shade in between and spans black, right through to white.
Black, grey and white are heavily symbolic colours too. Black is always associated with badness, black looks are threatening, fear of the dark, black Monday, etc. White is seen as pure, light and good (if used a little tongue in cheek at times). The baddies in films are nearly always dressed in black, like Lord Blackwood in the latest Sherlock Holmes movie - even his name contains the word ‘black’. Grey is associated with intelligence: brain matter, the grey head of wisdom, our man-made environment around us if you start to think about it is very grey in colour.
Why not try thinking about these associations the next time you work in monochrome, the results could be interesting. On a purely practical level, greys give perspective in a piece of artwork. The most straightforward example is a mountain range. As each mountain rising behind the one in front it is lighter in tone, heading more towards white, creating the illusion of space, perspective and distance.