If your book isn't in at least one category where it’s ranking #1, it is not going to do very well with any sort of promotion or future sales. This is because once you have your book in a #1 rank, it's recognized in the Amazon algorithm as a "Best-Seller."
A few days (or sometimes a week or two after the #1 rank registers) Amazon will put a best-seller banner next to your book, so when a reader types in a search and your book comes up, the banner is going to instantly grab their attention and they are much more likely to purchase your book instead of your competitors. This banner gives the readers the idea that Amazon has endorsed your book, which gives your book instant credibility.
You want to do all of this BEFORE a promotion, because you want to have that banner, and you want to be ranking as high as possible with as many reviews as possible.