Relaunching a site is often cause for celebration — you get to debut a fresh new design, plus (hopefully) give your users a better experience through improved navigation and functionality.
Common to many relaunches is the changing of URLs. Perhaps directory structures changed, or you transitioned from using underscores between words to Google's preferred dashes. What used to be:
www.mysite.com/attractions/outdoor/mini_golf.html
May now be:
www.mysite.com/outdoor-attractions/mini-golf/
So what happens when the new site launches and a user who tries to find something via a search engine ends up on an old URL?
The dreaded 404 Not Found error.