Secunia is one of a short list of firms that perform vulnerability testing. While it does not perform many of the tests that a Blink does, it is light and can be run from Secunia's web site. Most users are aware that Windows system updates are important -- or at least they don't interfere with the automated updating system -- but many of the new threats come from applications such as Flash and Acrobat. These programs are in direct contact with the Internet in a way that exposes them to all types of attacks. Few users would think to even check to make sure these applications are updated.
Secunia is also very useful to finding applications you may not have even remembered installing. On my system it found an ancient version of WinZip. It clearly was not something I was checking for updates, yet many attachments come as zip files and WinZip will automatically start up to uncompress the zipped file. If the attachment was dangerous, WinZip would have been compromised. So, a program like Secunia is important to keeping track of all the potential entry points to your Windows computer.