For those who follow such things, if your novel or audiobook gets placed on the 'also bought' listings below a popular title, it offers visibility, which translates to potential readers/listeners having the opportunity to notice and consider a reading/listening to a novel that they otherwise my never have come across.
It's advertising that can't be bought, but is worth a lot with respect to visibility. Relic Tech, the ebook version did very well with this on Amazon's Kindle, although it has faded from the first page of 'also bought' thumbnail listings. However, the audiobook version of Relic Tech has caught on and is appearing on the 'also bought' listing for a number of popular titles.
A neat one Relic Tech is paired with is a SF novel read by Luke Daniels, narrator of the Iron Druid Chronicles, which I discussed in a previous article. You can see here: Kevin Hearne and Luke Daniels: A Great Pairing for the Iron Druid Chronicles
How does one's works appear in the 'also bought' listing? There is very little an author/publisher can do, other than to write a darn good novel and do whatever can be done to provide an initial kick start upon publication. Then, it's up to the readers--their preferences, and if your novel tends to match those preferences as demonstrated through similar purchases. Thus, there's some timing and promotional efforts that can be done by the author and publisher, but really it's the Amazon/Audible algorithms and reader/listener preferences, combined with some luck, that make the difference.
I believe a good cover and title can help too. Maybe name recognition, (but I don't think that I have that).
I just found it pretty interesting and cool to be paired with Luke Daniels, even if indirectly.