No one technically needs a keyboard for an iPad. You can type on the screen. But still, it's inefficient. It's not tactile. And that keyboard eats up so much of the iPad's screen space. A dedicated keyboard can help for long-term iPad writing projects.
Keyboard cases are odd ducks, strange marriages. A case with a keyboard won't protect your iPad as well. Compromises are made. I usually recommend a standalone keyboard instead of a case.
Logitech's Ultrathin Keyboard Cover for the iPad is the exception. It's not a case so much as a Smart Cover with a keyboard; it's a remake of the simple, flat Zagg Keyboard Case, but updated for the magnet-friendly third-generation iPad and iPad 2. The $99 keyboard is more expensive than simple standalone keyboards, but it's equivalently priced to many keyboard cases.
Unlike many uglier keyboard cases, Logitech's ultrathin marvel looks as stylish as the iPad itself, matching the iPad's aluminum exterior on its outer shell. Snap the cover onto the iPad and fold it up with the Keyboard Cover, and the whole package feels like one organic piece, like an ultrathin laptop.
Snap the lid off and lay it down on a desk, and the iPad (whether iPad 2 or "new iPad") docks in a narrow white groove behind the keyboard. Another set of magnets inside locks the iPad in place when it's in a specific landscape orientation, volume buttons up. It's a strong-enough grip to keep the keyboard attached when the iPad is lifted, but the iPad can still fall forward if tipped, making this a keyboard for solid, flat surfaces only. While the extra magnet support works only in that orientation, you could also dock your iPad in portrait mode; I found that this worked, but it felt more tenuous.
You don't need to worry about the keyboard cover scratching your iPad, as rubber bumpers at the keyboard's corners prevent contact with the screen when it's used as a cover. It's not a Smart Cover replacement, though; the Keyboard Cover is thin, but it's not that thin.