If you look at the sidebar you’ll see a section labeled as Sponsored Links. It holds some AdSense ads. According to the AdSense Program Policies, publishers aren’t allowed to publish ads on non-content pages. Non-content pages include non-existent pages; ie. 404 error pages.
By setting the Section Specific Text Widget to hold my AdSense codes and display them everywhere except on 404 error pages I’ve just ensured that I am complying to the AdSense Program Policies.
Of course I’m sure you can think of thousands of better ways to make use of this nifty widget.