This is a very useful feature because you can make a walk animation that you can simply change the animation weight to see the character start running, because it will increase the scale of the movement of the bones in the animation. This will increase the hand and the leg swing as you increase the animation weight and the character, making it appear as though it is running.

Can easily switch sprites in between the animations like switching clothes and other multiple behaviors.

Watching and moving can be programed depending upon certain events.

Size as well as animation speed can be easily changed during an animation.

Can easily export to different texture sizes to target multiple resolution platforms.

Can easily change the Z-Order of a sprite during an animation and can also change the color mask on a sprite, in between an animation.

Create once and use the same animation for different characters just by simply cloning the same animation for the new characters.

You can nest skeletons and change the timeline of the animation in between the animations in order to make some cool effects.

There is an extension available for Adobe Flash users, which gives a WYSWYG design window inside of flash.