Guess from the day when you couldn't boot off of a cd-drive. You had to boot from the floppy that contained the drivers for the cd. Most computers can boot from a cd now days. The slackware distro boots into the install from the cd so there isn't a need for a floppy unless your computer is not capable . the second cd of slackware contains a Live cd that runs linux straight off the cd with out install, this is also a bootable cd.
read:
http://slackware.com/book/index.php?source=x209.html
it explains at the top.
