Spreadsheets are, in many ways, the application that launched the personal computer into the business world. The very first spreadsheet, VisiCalc, was created for the Apple II line of computers and was an immediate success. Lotus was the first company to bring the spreadsheet to the IBM PC compatible with Lotus 1-2-3, and other vendors quickly followed with their own versions of the application.
Spreadsheets are more popular than ever today, often being found as the cornerstone of what People Who Like Big Words call "Administrative Support Systems". Even a simple spreadsheets give a person enormous power to keep track of how money and information flows through a company, or to rapidly try out several values (for a product price or a test curve, for example) and see the ramifications of each one.
Spreadsheets, more than anything else, were at the heart of the first "look and feel" lawsuits. As more companies made spreadsheets, the question came before the courts: Can you copyright or patent the 'look and feel' of a program -- the way it appears to the user and the keys you need to press -- even though other people are writing their own programs from scratch? The answers have been ambiguous and mixed over the years, but the general idea currently is that you can copy another program's "look and feel" (within reason) without getting sued.