The important part of a threading lathe is the leadscrew. Many are 8 TPI and some of the larger ones are 4TPI. A fractionalized leadscrew and threading gears make it simple and easy to use a threading dial when cutting fractional threads. Many thread pitches let you engage the half nuts on any mark on the dial.

Try cutting decimalized thread pitches like are used with metric and you'll soon see the ease that using fractional pitches compared to decimalized as used with metric.

Many folks that try to cut metric threads will resort to keeping the half nuts closed and reversing the lathe on every pass = slow, slow, slow. Being able to use a threading dial makes the whole process much faster and easier.