13 Feb 12
26 Jun 12 10:24 pm
It logs 404 ("page not found") errors and helps you easily re-direct those to pages in your blog.
For one example this error could be from an old static website (which you've replaced with a WordPress blog), where the user has an old link to an *.html page, but WordPress has the same content in a WordPress static page (but no .html file name).
Or could be any other reason.
Or maybe you edited a "slug" or deleted a post and need to redirect that to the current post or page - good for those too. Pretty darn useful - and you don't have to understand .HTACCESS files or Apache Server Directives or any of the other arcane 301 redirect stuff.
This is all on a "page-by-page" basis. Usually to redirect a whole site, you'd do a 301 ("moved permanently") from the control panel of the hosting service of the old website - that would handle the whole site in one 301.