A conduit range is only useful if the whole ladder is there. An installer who can get 20 mm but not 25 mm will buy both from someone else, which is why the range runs 10, 16, 20, 25, 32 and 40 mm rather than stopping at the popular sizes.

Each step in the ladder has a job. 10 mm carries telephone and data wire, 16 mm is the smallest most electricians accept for a lighting circuit, 20 mm is what installers reach for by default, 25 mm is where a run stops serving one circuit, and 32 and 40 mm carry distribution.

Rigid lengths come with socketed ends so a run joins without a separate coupling at every joint, and cold bending lengths turn a corner over a spring instead of waiting for an elbow. Both share the same fittings, so a buyer stocks one set of accessories rather than two.