I'll add to this - if you work on a software project to port an excel spreadsheet to real software that has all those properties, if the spreadsheet is sophisticated enough to warrant the process, the creators won't be able to remember enough details abut how they created it to tell you the requirements necessary to produce the software. You may do all the calculations right, and because they've always had a rounding error that they've worked around somewhere else, your software shows calculations that have driven business decisions for decades were always wrong, and the business will insist that the new software is wrong instead of owning some mistake. It's never pretty, and it always governs something extremely important.