It can sometimes be difficult to erase a line that crosses another line. This is one of the many uses for an eraser shield. Eraser shields are made from thin sheet metal and are perforated with various shapes. The eraser shield is laid on top of the line to erase, shielding the neighboring lines that are to remain on the drawing.
A series of holes across most eraser shields is a convenient way to make hidden (dashed) lines. The line of holes is aligned with the line to be dashed and the spaces are erased away. This is a good way to make consistently spaced hidden lines.