Using heatmaps from the
https://usertrack.net tool on its own landing page I usually notice:
* What buttons in the header are actually needed/clicked/hovered https://i.snipboard.io/UShRbI.jpg
* What non-clickable elements users try to click on (eg. product images, they expect to get a lightbox, but nothing happens)
* If a specific UI element is visible enough and users know it's interactable
https://i.snipboard.io/nJ7AlX.jpg
I personally find the session playbacks a lot more insightful, because heatmaps don't really tell the whole story, but a session playback can show you in what order do your users read the page, what are they trying to do, what are their last actions before they leave your site, etc.