As avi said, it shows that you're pointing to it. This is obviously useful if you're not using the mouse, as others mentioned. But it's useful even when you are using the mouse, even though there's a cursor that ideally also shows what you're pointing at.
For example, if your cursor is on the edge of an element, it may not be obvious what's going to happen when you click. This is even more pronounced when the click target isn't exactly the same as the rendered object. (Is the boundary part of those input areas?)
So the highlighting saves needing to either look closely at your cursor and the element, or mousing more precisely to a point that's obviously over the element.
For an exaggerated demonstration, just try to upvote one of the comments on one of the posts on this page. The changing color is definitely helpful. Admittedly those are much smaller targets than an input area, and the highlighting serves to let you know something happens when you click there (which you already probably knew for an input area) but it's the same general idea.