Amazon keep moving them.
One day up the top, the next day down the bottom, no rhyme or reason to it.
But yes, also boughts are essential. I can tell which days the majority of my books have the AB slider at the bottom and which at the top, since being at the top improves my sales for the day.
But there are 2 things in play here.
First, the slider is full of other people's books, and this means your book is on theirs, most likely in the exact same spot. Which is advertising your book to that books readers for free.
The other thing is when all or most of the books on the first page of the slider are you own, and this pushes people just finding you into your existing books.
When a book is new, the slider has books form all over. As the book gets older, the first page becomes and more my own books. And this is good, because it means my back catalogue is selling, and every single sale back there is sheer gravy with icing on top.
People do go looking for the AB slider, wherever it is. and a lot of readers are aware of how much authors pay for ads, and try not to make us pay for their buy.
Personally, I consider the AB slider the most important part of the whole product page. After the product of course.