+/- adjusts the grid in binary steps. Simply instead of one block grid use half block grids. This allows you to snap to centers, or even smaller grids if you want finer control.
that only helps in some cases. if you use a block i.e. 0.25 x 0.25 it won´t help you. neither if the block is i.e. 0.57 x 0.93
But in my opinion the grids biggest use is to start a building in allignment with the world direction, so you are not building diagonal. But aside that i see almost no use for it other than causing confusion. Most people it helps in the beginning as its more Minecraft-like but once people discover the magnet function and free placement possibilities they take the Grid as standard and get confused when grid-system and magnet-system collide. They think its a bug in the game and get annoyed. And instead of learning the awesome building system of RW they keep using the Grid because its the path of least resistance, but they do get frustrated because they are not able to create the really detailled buildings and constructions of others.
I see the grid as a kind of learn-to-walk-help. But once you can walk you should not rely on it anymore.
I know that others, i.e. Deirdre, still use it and are true masters in building, though i admit i do not know how and to what advantage they use it.