This cannot be stressed enough. I've outlived generations of finance teams at many startups, and I've seen firsthand the masterpieces/abominations left behind in Excel. Imagine a dozen sheets with ad-hoc queried data copy/pasted from System A/B/C/D into Excel, with formulas that feed formulas that feed formulas. Sometimes columns are inputs (seasonality adjustments for monthly forecasts), sometimes their outputs (modeled growth * last year * seasonality adjustment) and more often than not their right next to each other and maybe they have different cell background colors or a black separator line. Maybe.

And this is just finance. For many e-commerce businesses, planning is done in Excel with equal zeal.