Imagine walking into and out of a supermarket. If you did not purchase anything, the supermarket managers probably didn’t even know you were there. If you purchased something, the supermarket knows something was sold but that’s about it.
Visiting a website is a radically different proposition if you look from the lens of data collection. During the visit to a website, you leave behind a significant amount of data, weather you buy something or not.
The website knows every “aisle” you walked down, everything you touched, how long you stayed reading each “label,” everything you put into your cart and then discarded, and lots lots more. If you do end up buying, the site manager knows where you live, where you came to the website from, which promotion you are responding to, how many times you have bought before, and so on. If you simply visited and left the website, it still knows everything you did and in the exact order did it.
With this kind of information, imagine the kind of improvements you could make over time to help your website grow.