Joel Spolsky wrote a book called User Interface Design for Programmers (back in 2000), and he put some of it up on the web. Chapter 9 includes an amusing anecdote about how software gets used in the real world by real users:
In the days of Excel 1.0 through 4.0, most people at Microsoft thought that the most common user activity was doing financial what-if scenarios, where you do things like change the inflation rate and see how this affects your profitability.When we were designing Excel 5.0, the first major release to use serious activity-based planning, we only had to watch about five customers using the product before we realized that an enormous number of people just use Excel to keep lists. They are not entering any formulas or doing any calculation at all! We hadn’t even considered this before.
I’ve also read that the most popular database software in the world is Excel — even though, of course, it isn’t even a database management system.