The main purpose of seam carving is to reduce the width (or height) of an image without scaling (squishing) the image.
This effect calculates the line through your photograph with the least amount of changes and deletes that line shifting all pixels over to fill the gap. This is often called "content aware resizing".
This effect is designed to be used with natural photographs. It is not designed to be used with an active selection. So, before running the effect, press Ctrl-D t