Holding a Tablet, and eReader for hours can be kind of tiring since it stills require some strength to hold it in your hand with one hand.
A tablet stand like Targus Mini Stand Designed for Apple iPad 16GB, 32GB, 64GB WiFi and WiFi + 3G, iPad 2, Motorola Xoom, Samsung Galaxy AWE65US (Clear) also looks nicer and is more easy to keep your tablet standing up then using a box, and phone book to lean your Tablet against to keep it up right.
If you use your tablet like a laptop with a USB or bluetooth keyboard then using a stand will help you hold your tablet or ereader while you use both hands to type.
Using a stand help you more when drawing, and painting with Apps on your Tablets since it would not move while you draw on it.
A Stand might help you with stationary games like Angry Birds, Fruit Ninja, Cut the rope, etc since you do not need to hold it, and could just use both hands to play the game since you are not holding your Tablet.
Using a tablet stand to read can also help you when reading speeches since reading a speech while always holding a tablet and looking down on it can look kind of awkward.
A Tablet stand will be best for watching very long movies, viewing a lot of pictures, or listening to music since you don't have to hold your tablet for hours while you watch a video.