Whether you've enrolled your book(s) in KDP Select or not, the conversations around changes to the popularity lists affect you too. Why? Because Amazon's recommendation engine is based on rankings, both on the bestseller lists and the popularity lists.
Most of you are familiar with the alsobots (the list of books that customers who bought your book also bought and that are similar to your book in genre, price and ranking). The alsobots are what you probably thought of first when the reco engine was mentioned. And they're important, of course. But how about the emails that Amazon sends out periodically based on your browsing habits? Those along the lines of:
Customers who have purchased [specific genre] books on Kindle might be interested in this month's best sellers or related books Recommended for You.