As usual the big boys are always at the forefront on experimenting with new web strategies...
If you visit the amazon.com website now (on desktop), you will likely see a module in the center of the homepage titled "Amazon wonders...Are you a lover or a fighter?" This may actually be a data collection element for dynamic web personalization.
On the surface it looks like you are participating in a simple survey: after you have selected an answer it shows you the percentage of people who have chosen either answers (social proof...), then immediately it recommends you related products based on your input.
We suspect they may have added this new data point to your shopping profile (if you are logged in) or through a cookie. So you may be seeing more of either category of products all over the website on your subsequent visits. Ka-ching!
Principles of Influence: Social Proof (see 6 key principles of influence by Robert Cialdini)
The percentage bar is a simple "social proof" element, it shows there are others who have participated in this survey (not just you!) and it shows how your input benchmark against others. If they launch another of such survey in future, you are highly likely to participate again because you want to find out how you benchmark against other shoppers.
Simple yet subtle way to collect data!