The GridOn command enables a grid on the graph screen (you can disable it again with the GridOff command). How fine or coarse the grid is depends on the Xscl and Yscl variables. Drawing the grid just involves plotting points all the points of the form (A×Xscl, B×Yscl) that are in the graphing window.
GridOn can be used to shade the entire screen if Xscl and Yscl are small enough that the points on the grid are one pixel apart:
:ΔX→Xscl :ΔY→Yscl :GridOn
This is one of the shortest ways to shade the screen, although Shade( can be used for a (usually) even shorter way. However, using GridOn is also very slow: the fastest way involves the Horizontal or the Vertical commands in a For( loop.
You could also use GridOn to draw the playing grid for a Dots and Boxes game.