It describes where to find all the directories on each partition, and, where to go on the disk, to find out which files are in each directory. By corrupting or destroying any one of these three items, a virus can make a disk completely unusable. Moving the Partition Table to a secret hiding place and replacing it with its own code. Preventing any program from writing data to these parts of the disk. Preventing any accidental booting of your computer from a floppy disk.
Most viruses still enter a computer from a floppy disk, or other removable media.
If desired, requiring a password before your computer will boot to and access your hard disk.