1. The occasional heavy game performs better
2. iMovie editing performs better
3. 4G compatibility (in countries where the 3 doesn't)
4. Because I want the latest
The newer OS can kinda bog down the older iPads, even though they work. The new unit will be MUCH faster for you with "buttery smooth" graphics. If you we're happy with the older iPad all this time this new one will keep you happy even longer as I think they are kinda hitting to ceiling on what you can do with a tablet.
Smart Case? It all depends on how you use your iPad. Some kind of protection for the back is nice so you do not need to worry about scratching it when you lay it down. There are a lot of case options and more to come. I choose a Targus leather case that clips on the device without it extending onto the bezel. It has a fold over cover that not only protects the screen while laying on the table, but it also turns off the screen with the magnet. I have and tried the Apple smart case but went with the Targus out of personal preference.
If you go to the accessory forum for iPad, you will get all kind of info.