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It's really useful if you know even a single other person with an android-powered phone. Essentially, it allows you to share a url/contact info/application/bookmark/clipboard contents with just a couple clicks.
Once you've got the barcode scanner installed, launch it (don't be surprised when your camera launches - afterall, that's how it snags the barcode!) and select the menu button and choose SHARE to share a contact/url, etc.
Eventually, the hope is that you'll see QR codes (google's barcode format) everywhere. e.g., You visit a restaurant that has a QR code on their menu that offers a coupon/discount. There are several android app sites that offer QR codes (surf for an app on your desktop, scan the QR code on your monitor with your nexus one) if you prefer to surf for apps on your desktop rather than surf for apps in the market on your phone.
Now, here's what I'm getting at: Why settle for sharing QR codes in-person when you can also share over the 'net? I ran across this site today which allows you to generate your own QR codes online.
If you've already got the barcode scanner, here's a sample:
