When a site rebuild is launched the content hierarchy most likely changes. That means all old URLs will become 404 error pages, their built up SEO is lost, and the new urls start off with no seo value.
Instead of tossing out that built up SEO, the 404-301 tool converts 'page not found' (404) errors to 'page has moved' (301) redirects.
Browsers will automatically load the new url for the content, it’s transparent to the users. When search engines hit the old urls they will refresh their index with the new urls and also transfer SEO from the old urls to the new urls.