Over the last few years, ReS₂ has generated a myriad of unattended queries regarding its structure, the concomitant thickness-dependent electronic properties, and its apparently contrasting experimental optical response. In this paper, with elaborate first-principles investigations, using density functional theory (DFT) and time-dependent DFT, we identify the structure of ReS₂, which is capable of reproducing and analyzing the layer-dependent optical response. The theoretical results are further validated by an in-depth structural, chemical, optical, and optoelectronic analysis of the large-area ReS₂ thin films, grown by the chemical vapor deposition (CVD) process. Micro-Raman, x-ray photoelectron spectroscopy, cross-sectional transmission electron microscopy, and energy-dispersive x-ray analysis have enabled the optimization of the uniform growth of the CVD films. The correlation between the optical and electronic properties was established by static photoluminescence and excited state transient absorption measurements. Sulfur vacancy-induced localized mid-gap states render a significantly long lifetime of the excitons in these films. The ionic gel top-gated photodetectors, fabricated from the as-prepared CVD films, exhibit a large photo-response of ~5 A/W and a remarkable detectivity of ~10¹¹ Jones. The outcome of this paper will be useful in promoting the application of vertically grown large-area films in the field of optics and opto-electronics.