Google released its crawl error feature in Webmaster Tools some time ago (back in 2006), but since it's release webmasters have wanted to know the exact sources of the URLs causing the errors. Well, now it's available. This is important for those involved in SEO for a number of reasons.
The feature will help to identify and track down the cause of a particular "not found" error. This in the end will improve user experience by showing fewer dead-ends and give webmasters the ability to prevent it from happening in the future. If you happen to get those not found errors corrected, you might even find that there is a boost in ranking thanks to additional incoming links. For "not found" errors from your own site, webmasters will be able to put redirects to appropirate URLs in place.