They will use it both as intended, a spreadsheet for calculations, usually for reports.
But also as publishing software (think InDesign) and format their spreadsheet to look like an invoice / quote, a hand made Gantt chart (yeah with arrows as delimiters, as opposed to filling the cells), etc ...
It's both a fascinating and horrific thing to behold, something that wouldn't be out of place in a H.P Lovecraft story.
We're going to try to get the Gantt folks to modernise a bit and use a dedicated tool for planning but we already know it's going to be complicated to get the senior folks to even consider it.