I have a network meter on my xfce taskbar, and it shows uploads and downloads whenever I move my cursor on a website I view with a JS enabled browser. It's almost all of them.
Hang on - the one time correlation does strongly suggest causation is when trials include randomized interventions, which i pretty much the case here (once in a while, move my mouse and see if network traffic spikes).
As somebody who worked in the past on a piece of software that generated heatmaps from cursor movements on websites, I can confirm that it's a very widespread thing. Well, it was ~5 years ago, so I'd guess it's even worse now.
For a non-nefarious use case, it can be used to iterate on the UI to create a better user experience because it can expose areas that people aren't seeing on the webpage. Your site might have the important content or useful navigation in a place that users aren't noticing which causes them to leave the site in frustration.
Yeah, basically this. The data was aggregated and a graphical heatmap was displayed on top of the website, with some fancy accomodation for responsive designs. You could see heatmaps for hovers and for clicks. Customers then optimized their shop flow, adjusted graphics that looked like they were clickable but weren't, moved important content into more visible places, etc.
(You can always open the browser inspector and check network traffic for each page or, if you are using Chrome, dive into chrome://net-internals/ )