If you had to repair your linux installation, and booted up with a rescue boot disk, you could use chroot to change from the present root directory in ram disk to the disk partition that is normally your root directory.
Another use is to run services like apache from a different root. This way, if it crashes, your system isn't exposed. The new root dir will contain a scaled down copy of the system heirarchy. Just what is needed to run the service but no more. You may also use the --restricted option for the bash shell.