When Nintendo released the DS, they said that it was not a successor to the Game Boy Advance (GBA) hardware, that it was Nintendo's third pillar (the other two being the GameCube and the Game Boy Advance). Then why was a GBA slot included on the DS? Certainly, the ability to play GBA games is a major selling point of the DS. The GBA library is huge, and if video game history has taught us anything, it's that backward compatibility is a feature that gamers want and are willing to pay for.
In addition to playing GBA games, the GBA slot on the DS allows Nintendo to release expansion paks. Since DS games run on cards, these expansion paks can be used concurrently with the games. The first expansion pak released for the DS in the US is the Nintendo DS Rumble Pak (at least one other has already been released in Japan). This pak, as its name suggests, rumbles at appropriate times during gameplay — which is something we've come to expect from gaming. Of the gaming systems currently available (Sony PlayStation 2, Sony PSP, Nintendo GameCube, Nintendo Game Boy Advance, Nintendo DS, and Microsoft XBox) only the PSP and the GBA lack rumble capability.
The rumble isn't as powerful as it is in a console controller. It is manufactured into a GBA pak, after all. It is, however, well executed. The rumble pak provides tactile feedback without getting in the way of the game which is important. If the pak shook too much, it would shake the screens as well, and this isn't something console developers have to worry about. One can hear the rumble pak start up and wind down, but if a game is being played either with the sound on or headphones on, it isn't all that noticeable.
The Nintendo DS Rumble Pak is available as a pack-in with Metroid Prime Pinball or as in individual purchase for around $10.00, and Nintendo has said that Mario and Luigi: Partners in Time (the sequel to Mario and Luigi: Superstar Saga) and Mario Kart DS will also make use of the rumble pak. The only downside to using the rumble pak is that it occupies the GBA slot. As Tycho of Penny Arcade said, "if I ever take the DS someplace I always put a GBA favorite in that slot¹" because carrying two games without carrying any cases just makes more sense. Games that use the pak are also playable without it (so far), so you don't need to rush out and find one. It adds good feedback, and I like it, but it is wholly unnecessary.
- Penny Arcade October 19 news post, Jerry Holkins, http://www.penny-arcade.com/news.php?date=2005-10-19
Thanks to althorrat for informing me that Mario Kart DS will use the rumble pak and that the rumble pak is the first US expansion, not the first expansion worldwide.