Facing is a lathe operation in which the cutting tool is fed radially (perpendicular to the axis) across the end of a rotating workpiece. As the tool travels from the outer diameter toward the centre (or vice-versa), it removes a thin layer of material and leaves behind a clean end face.

Why ensuring a flat surface perpendicular to the axis is correct:

• Facing generates a flat, true end surface that is square (90°) to the rotational axis of the part.
• In assemblies, components are located and clamped against these end faces. If the face is not flat and perpendicular, the mating parts will sit at an angle, causing misalignment, uneven load transfer, gaps, or run-out in the assembled unit.
• A properly faced end also provides a reliable reference/datum surface from which further lengths are measured and machined.

Why the other options are incorrect:

• Improving cutting tool life relates to cutting speed, feed, coolant and tool material — not to the geometric purpose of facing.
• Reducing surface hardness is a heat-treatment effect (e.g. annealing); machining generally work-hardens rather than softens a surface.
• Increasing overall strength is a material/heat-treatment property; simply removing metal from the end face does not add strength to the workpiece.