Scrat is the entire appeal of Ice Age. He's a ridiculously well done character for worldwide consumption: there's no language barrier to get him at all. So they can make a short film about him trying to get his acorn and play it everywhere without any localization. It doesn't matter if the film itself isn't so hot, the handful of Scrat sequences throughout will make it work. And any one of those sequences can be played as a trailer. I actually look forward to the next Ice Age film because even though I have no desire to see it (I've only seen the first, in fact), it means there will be Scrat shorts as trailers.
Scrat is awesome. He's one of the best characters created for film ever.