Why boot to a cd?
For loading an OS or an installer to load an os to the disk.
It holds 650MB's (and more if you buy the right cd's) instead of 1.44MB's.
It's many, many times faster than a floppy is.
The old way to load an os is to boot a floppy that has drivers for running the cdrom drive and you install from the cd.
This way the boot files are on the cd, you boot from the cdrom drive and install from the cdrom drive.
How? you go into the bios and look for the boot order, set the cdrom to be the first boot device. Older systems dont have this option.