This is useful if you draw or model in 3 dimensions but wish to view the drawing in 2 dimensions. Also useful for drawings created as a composit of 2 drawings (1 inserted to the other) which were not created on the same plane or in the same UCS/WCS. Lines drawn between 2 such drawings ofton look to be in 1 plane when viewed in 2 dimensions. But, are really "cats-cradle" constructions when viewed in 3 dimensions. Also useful, when exporting a drawing to 3DStudio.