HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 470nm to 2450nm with resolving powers from 3300 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes -SCAO (including a High Contrast capability) and LTAOor with NOAO. The project is preparing for Final Design Reviews. From the perspective of data reduction, HARMONI introduces unique challenges due to its multiple adaptive optics modes, four spatial scales, eleven gratings, and two distinct NIR detector read-out modes. Capitalizing upon CRAL's experience in developing instrument simulators, this complexity prompted the development of the HARMONI Instrument Numerical Model (HINM). Built upon standard astrophysical Python frameworks, this software uses the Fourier optics concept to propagate a wavefront through the instrument, and leverages existing simulation tools for adaptive optics, sky and detector simulation. This enables the generation of synthetic detector read-outs for both calibration and science exposures. This paper highlights the crucial role played by the HINM simulator to develop the data reduction pipeline and elaborate instrument calibration procedures.