As far as I remember the A and B drives were used for floppy disks. In the olden days home computers didn't have hard drives, and typically either had a single or dual floppy drive. With a dual drive you could do fancy things like make backup copies, without having to repeatedly store data to RAM and swap source and destination floppies. In the 1980s home computers typically didn't have enough memory to store the entire contents of a floppy disk in RAM, which made the process of creating backups irksome if you didn't have a dual drive.
If you've only started using Windows based computers within the last five years then the missing A and B drives may seem mysterious. Floppy drives started disappearing from first laptops and then desktop machines in the early 2000s.
I was working at an Apple reseller when the iMac came out. Part of our sales pitch was to try to convince everyone who bought an iMac that they needed a USB floppy drive to go with it. Everyone was convinced that people needed that floppy drive.
Floppy disks were also used in home computers that had hard drives. Before it was popular to have local networks, and before the internet was commonplace, sneakernet was the best way to share files among computers. https://secure.wikimedia.org/wikipedia/en/wiki/Sneakernet
If you've only started using Windows based computers within the last five years then the missing A and B drives may seem mysterious. Floppy drives started disappearing from first laptops and then desktop machines in the early 2000s.