When you draw something in 2D graphics software you can put a grid on a sheet, so that when you turn a line you get only right angles and straight lines instead of diagonal.
When I use knife to cut surface I would like to move knife by precise straight line and right angles, so would be good to have some grid on the surface that being cut (may be addon).
If you press C after hitting K for the Knife tool the cuts will be constrained to right angles and 45 degrees. This is relative to view.
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