Use the grid when you want to layout elements with regular sizing and spacing, for example panels on a page.
Enter your desired intervals and click OK.
The grid is updated to reflect the new values.
Now as you drag and resize elements on the page their positions will jump to the nearest grid line.
For best results make sure your elements are sized to fit the grid before positioning them.
To center the grid on the page go to the Preferences/Options and under the Ruler tab check the "Place origin at center of ruler" option. See Preferences | Options for more information.