What's the "spreadsheet from hell" that’s currently holding your business together?

I have like many many of those massive, messy, 100 tab Excel or Google Sheets that basically run the entire department because there isn't a dedicated software for the job.

I’m looking for ideas for a new SaaS project. If you're using a complex spreadsheet to solve a problem that specialized software should be handling, I’d love to hear about it.

what is the spreadsheet tracking, and what’s the biggest pain point of using it