In Blender, hovering the cursor over a UI item highlights it. This is quite nice, because it's clear which item will be interacted with when clicked.
However, some of the UI widgets don't give proper feedback when hovering the cursor. For example, number fields highlight the entire widget when hovering over it, even though it consists of three distinct areas (left arrow, text field, right arrow). The problem is that the current highlighting gives a false sense that clicking the button anywhere does the same thing. This can lead to user error and confusion when attempting to interact with these fields.
Also, when buttons have no background (icon only), there's no highlighting and pressed state. This makes it unclear if a button has been pressed or not.
Use rollover to hide/add useful feedback:
Highlighting can also be added more places, such as the headers in the Properties editor:
(click to view)