Recall that the Rules of Thirds encourages to to you mentally divide the frame into 3 layers from top-to-bottom, and 3 layers from left-to-right. And, having done so, to place the subject(s) within the layers, or on the grid lines.
Occasionally in so doing, your photograph will wind up with a void in 1/3 or 2/3 of the photograph that "feels" unbalanced or un-anchored or unsatisfying.
Adding a Balancing Element can sometimes address this tension and "resolve" the photograph.
Consider for example, how the photograph "Light at Sodus Pt" herein would "feel" with and without the lighthouse at at the end of the pier.
In the 3 images shown here from the Fabius Farm Season
series, the small tree, geese, and sun respectively, provide the Balancing Element.