It lets you save your game (ala Game Action Replay for NES) and has the memory for probably a couple hundred save slots. It can also save to blank Famicom disks instead (for the Famicom Disk Drive). You can save your game with cartridges or disks. To save cartridge saves to a disk though (instead of directly to the unit), you need some special device called a Master Link. I have no clue what this is, but there is a 15-pin Comm port on the back of the unit (it looks just like the one on the front of a Famicom). Any ideas what a Master Link is? Maybe just a cable....
Thanks to Rob Budrick for the info.
Info by Kyuusaku: Looking at the pics, one is a Game Master, the other is a Game Master BOY. GM is an actual copier, GM BOY is a Game Master missing the original GM's core IC which does the copying, thus the GM BOY can take advantage only of the GM's special features which include copying FDS disks and real time saving and making replay videos.