The Grid option allows the user to display a regular grid of points or lines over the 2D views (e.g. Opening Section View, Perimeter Support Designer, End Support Designer). To turn the grid on or off, select Grid from the View menu, and toggle the Show Grid checkbox. Grid settings can also be changed (Grid Style, Spacing, and Colour) in this dialog.
If the Snap option is enabled, then the grid intersections can be "snapped" to. You may find this useful when defining or editing the Opening Section.
Since the grid is always the same size (in world coordinates) in all views, this will help give you a sense of scale, if you have multiple views open, and if one view is "zoomed-in" and another view is "zoomed-out".
The grid will eventually "disappear" if you keep zooming "out" on the Opening Section. This prevents the grid from becoming so dense that it hides the model.
TIP - the F7 function key can also be used to toggle the grid on and off.