The Grid option allows you to display a regular grid of points or lines over the model. 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 graphically adding boundaries, support, etc.
Since the grid is always the same size (in world coordinates) in all views, this will help give you a sense of scale if one view is "zoomed-in" and another view is "zoomed-out".
A shortcut for turning the grid on or off is to left-click on the word GRID on the status bar.
The F7 function key can also be used to toggle the grid on and off.
The Grid Settings dialog can be accessed by right-clicking on the word GRID on the status bar.
The grid will eventually "disappear" if you keep zooming "out" on a model. This prevents the grid from becoming so dense that it hides the model.