The spreadsheet program that is part of Google Docs is nowhere near as feature rich as a desktop program like Excel or OpenOffice Calc. But if it meets your needs, you can do something that is awesome and that you cannot do with the other programs. You can share your work with others, securely, and let them work on it at the same time. And if you want to publish your spreadsheet as a web page, you can do that with a few mouse clicks. Here is an example.
Notice that a spreadsheet is really a little database. It has rows (records), and columns (fields). It holds data in a structured way. In the case of the example that I give here, you can see that by having your spreadsheet on the internet, you also have interactivity. There are links that the viewer can click on. Think about what is happening here. A spreadsheet has been created with information I want in it, it's published on the Internet so anybody can see it, and it has interactivity. When there are changes made to the spreadsheet, those changes show up on the web automatically. This goes way beyond what a spreadsheet traditionally does.