Daily weather forecast relies on the precise measurement of weather parameters. The science of numerical weather prediction aims to describe the daily hydrologic cycle in numbers - humidity plays an important role in this - data errors will multiply during calculations.
Humidity and temperature values influence weather calculations e.g. through the water vapour balance equation - this formula expresses the influence of humidity through rain & condensation, and vice versa.
Incorrect measurement or incomplete humidity data directly leads to wrong predictions of a huge number of weather phenomena; this can include the condensation altitude of clouds, locations of hyetal regions, fog layers and storms.