If you have a lot of pages indexed in search engines and decide to change a path in your web site, you probably want to redirect from the old URL to the new URL.
If your Ubuntu installation locks up as the graphical boot screen attempts to initialize, and you can't get access to your system after starting it up, booting into a previous version can be a help.
This can happen after updating to the the latest update to Linux 4.4.0-108 kernel, which included a fix for the Meltdown vulnerability.