A new computer is essentially blank.. no operating system, nothing. You start it up and you can't do anything. In order to do anything, you need an Operating System. In the old days, before harddrives, floppy disks had entire operating systems on them, and were inserted when the computer was staring up, and when it went looking for an Operating System to start, it would boot off the boot disk, and you would have a working OS.
Fast forward to today.. you can use floppy disks to boot into OS installs, or if you need to boot outside of the OS to do a task, like flashing BIOS, or recover the OS. However, bootable CDs have become much more popular for most things, in fact, Macs haven't been shipping with floppy drives for the last few years.