Actually, Andrew Loomis talks quite a lot about this in his book on illustration. The idea is that you would line up elements in your composition to these lines. The lines you have here are symmetrical, not always the most interesting arrangement. Loomis shows how to divide up the composition in a nonsymmetrical way, which tends to be more interesting. It also makes it easier to divide up a square or other shape compositions. If you notice, the lines create a division in thirds, which is a common compositional devise. The place where the lines cross is supposed to be the best place to put your elements. To be honest, I find these devises to be a little too much like math, which is my mortal enemy. I tend to go with my gut, but I think this is good for you to play with and try to understand, then adjust and adapt it to your needs. Hope this helps :)