@martinibuster clickthroughs from SERPS is definitely another factor but what I feel is clicks on a conversion unit embedded in a web page might also be a factor. For Ecommerce sites this could be the Buy buttons etc., for blogs this could be the clicks on the submit button within a comment form, for download sites this conversion unit could be the download buttons or links and so on.
This would help them in arriving at the conversion rate which is yet another indicator of user satisfaction.
Don't you think this could at-least be true for transactional queries? But my guess is this might as well be used for informational queries as well.