If there is only computer program that changed the world, it would be VisiCalc, the first electronic spreadsheet program. It came from a student at the HBS in 1978-1979.
Previously when companies were planning to do acquisitions or disposals it was a long laborious task. Every time an assumption changed, everyone would have to go back to the calculators and see how that change effected every other factor. At best the process took days if not weeks. And, the assumptions constantly changed.
With VisiCalc, you just plug in the new number and in less than a second, you had all the answers. It was God’s gift to the financial world. Suddenly you could do a thousand “what if” scenarios painlessly. Nothing was the same after that.
And, since the program was run on Apple 11 personal computers, it made the PC an essential business tool rather than something to write letters and store recipes. And for IBM, the sudden appearance of Apple in their backyard, the business world, forced the company to produce its own PC.