It claims on the packaging it's a cheat system for the DS/DSi, and that may make you think it's like Gameshark and other cheating devices. It isn't.
This product lets you copy/overwrite save games on your DS cartridges. They have a database full of save games from other people that you can download and put on your DS cartridge, but there's no real way to actually cheat at your own game.
However, the ability to upload & download your save games means you can use other tools to modify them. With my Pokemon Platinum/HG/Black/White games, I'm able to copy my game to my computer, use shunyweb.info to convert it into a .SAV format and then use pokegen to modify the game before converting it back to .GDS and putting it back on my DS card. So for that purpose it is useful (although a bit slow).