I think book trailers exist because they can. In other words, it's so easy these days to pull together digital media into a really decent-looking short film and then post it on YouTube where it can be seen by everyone... so why not? Of course, it's the book itself that will actually do the selling in the end. The Harry Potter books sold zillions of copies without a trailer, for example, or an 'author's platform' (I'll discuss that some day, too), or a Facebook page, and there are thousands of other examples as well. In the end, will a trailer sell a book? No. Are they fun nonetheless? Absolutely! Should an author make one? Well, that depends.
If said trailer keeps an author from writing, the answer is a definite NO. If the trailer can be done without distracting the author, sure, why not. I happen to really like trailers, but honestly I don't choose to buy a book on the basis of a trailer. I imagine I'm pretty typical in that regard.