Wuld Nah Kest (i.e. Whirlwind Sprint)
I use it to travel. For example, I can travel to Whiterun and within a second of the city loading, be opening the door to my house. It is also great for just moving around the countryside/dungeons and getting to vendors quickly when encumbered.
I use it offensively. As a 2h wielding warrior, I need to close the gap on my opponents quickly and chop them, lest I get kited by silly frost mages.
I use it defensively. If I am getting owned (rare), there is nothing better than simply not being in the same place as the enemies that are trying to kill you. Instantly.
Additionally, it looks and sounds awesome.