I'm sure a spreadsheet works fine for many people, and from the comments you can see that others have used them for this purpose. Here are some advantages this site has:
* The data to capture has already been figured out for you. When you're first looking for a job you might not really know what to record about each listing.
* The approach has been figured out for you. I did not follow the GTD-esque approach initially, and I'm sure most other people don't. That in itself is useful, as is the site's ability to facilitate it.
* There is a bookmarklet for adding job listings. This saves you time.
* You can subscribe to an ical feed of your next actions, which you can't do with a spreadsheet.
* You can subscribe to RSS feeds from your favorite job sites so that you can easily review new postings in one place.
If you're ever looking for a job, please do give it a try and decide for yourself whether it's better than fiddling with a spread sheet :)
Seems like a solution to a non-existent problem.