A great Planet Money piece on how the spreadsheet got invented and how it entered accounting and finance. (Two versions at the link.)
The basic outline is paper spreadsheets existed before, then the computer made it easier to recalculate when changes happen. 400k accounting clerks have been reduced from the national workforce; while 600k new accountant jobs have been created because companies need more than the numbers now and want the what-ifs.
The really big change is you can now do what-ifs — models and sensitivities. And beyond accounting, that changed finance, all of Wall St and certainly business strategy.