Typed arithmetic allows to write expressions in CSS such a calc(10em / 1px) or calc(20% / 0.5em * 1px) which is useful for e.g. typography, as it allows to convert typed value into an untyped one and reuse it for number accepting properties or futher multiply the unitless value by some other type to e.g. cast from pixels to degrees.
Currently, authors have to use tan(atan2(length_value, 1px)) trick to get rid of unit information. With typed arithmetic authors can just do (length_value / 1px) to get the unitless value.