The crucial point is that interactions change a web page’s state - which means they change elements’ visibility, attributes and these kind of things.
Web Recorder records your interactions on web elements by first locating the element and identifying the action, only then a corresponding test step appears.
On the other hand, Spy tool merely spy your web page - which means it merely gives you elements’ locations on your web page and doesn’t record the action.
Of couse the trade off is that you can’t record interactions with Spy Tool.
Hope that it helps !