It does everything, badly. Which is why it's popular. People can get by without properly thinking things through, just jamming a few formulas on a spreadsheet, and patching the calculation when something comes up. You're never forced to think rigorously.
You can use it as a crappy database. Or a crappy UI. Or a crappy place to call external DLLs from. It's the perfect tool for that guy who "has a great idea, but just needs it coded up".
If a guy comes to you with an Excel question, he wants you to help him. Contrast this with a range of other technologies. If a guy comes to you with a question about c++, you tend to be helping each other. Same can be said for any number of things that non-techies would not think to ask. When was the last time someone asked you about Haskell or Erlang where you were treated merely as a means to someone else's end?