Its scale is divided by pi. Because circumference equals pi times diameter, dividing the scale by that factor means the number read at the overlap is the diameter directly. It removes both the arithmetic and the need to find the true centre line across the object.

Because any line across a circle that does not pass exactly through the centre is a chord and reads short, and the centre cannot be judged reliably on a large pipe or tank. Calipers large enough are also impractical. Wrapping the circumference requires no judgement about where the centre is.

A precision diameter tape, generally in stable steel with a vernier scale and a defined application tension, capable of resolving diameter on large cylinders to a few thousandths of a millimetre. It is used on pressure vessels, turbine components, rolls and large machined parts that no caliper can span.

Most often the tape sitting at a slight spiral rather than in a plane perpendicular to the axis, which measures a longer path than the true circumference. Excess tension on a stretchy tape, or debris and paint build-up under the tape, also read oversize. Perpendicularity is the usual culprit.

On precise work, yes. Both the steel tape and the component expand with temperature, so a measurement taken hot differs from one taken cold. Precision measurement records the temperature at which it was taken, and comparisons between readings should be made at comparable temperatures.

Through a defined tension applied by a spring or tensioning mechanism built into the tape, rather than by hand. Consistent tension is essential for repeatability at fine resolution, because variation in pull changes both stretch and how closely the tape conforms to the surface.

The same principle in a much simpler form - a tape divided by pi, wrapped round a trunk to read diameter at breast height directly. Precision is far lower and appropriate to the application, and the tapes are usually fibreglass rather than steel for durability outdoors.