The Time-Domain Extragalactic Survey (TiDES), is an upcoming spectroscopic follow-up survey for LSST transients and their host galaxies. TiDES has secured a quarter-million fibre hours on 4MOST which will result in the largest ever sample of spectroscopically confirmed Type Ia supernovae (SNe Ia) to use for high-precision cosmology, as well as a large AGN reverberation mapping experiment. I will present the current state of SNe Ia cosmology, and my work creating mock catalogues for the TiDES survey. I will detail how a sample of this size will revolutionise SNe Ia cosmology and why spectroscopic follow-up of transients is essential in this new era of big-data surveys.