LiveCDs can be a big help when you need to do a task that cannot be executed from within OS for some reason. Resizing partitions, changing passwords and firmware updates – is just the beginning of the list. It is especially welcome when your machine cannot boot from network ( PXE ).
There are lot of very good options like Knoppix, Parted Magic or LiveCDs of popular distributions. But there are cases when you need custom LiveCD with your scripts, packages and preinstalled software. Ability to modify root partition using writable snapshot on livecd, allows you to add software and modify configuration on the fly.