It has become common practice for web developers to install Google Analytics on their site to track traffic and visitor stats. However, another great tool that Google provides is Google Webmasters Tools and it is just as important, if not more, simply due to the comprehensive data it gathers.
Once you submit your sitemap, you can see how many urls have been submitted vs. how many have been indexed (there are many reasons why not all url’s get indexed).
You can also see all the Crawl errors, which are broken into 4 sections: server errors, soft 404, denied access and not found. From here you can view when each error was first detected, last crawled and even where the link is coming from. This is exceptionally useful when doing 301 redirects, especially when the errors are from an old/outdated source that you have no control over.
Sites that have huge traffic and have been around for years end up having all sorts of links all over the web and without 301 redirects, a great deal of users would get the mostly unhelpful 404 page. By doing proper 301 redirects, you avoid loss of website traffic and user frustration.