When a fill executes, it does so in sections. This is driven by the shape of the object, and where the Exit point is located. When the fill has to travel between those sections, it can either run under the rows of stitches that haven’t been filled yet, or it can run along the outside edge of the shape.
If the fill is a normal density, even, one of the lighter ones previously discussed, the traveling runs probably will not be visible and certainly not noticeable, so the most efficient thing to do is let the fill travel normally.
There are uses for fills that have much wider spacing between rows; trapunto effects for example. In a fill that has low density – where the stitches are really far apart -- you may want the traveling stitches to go along the outside of the shape, rather than under the rows where they would interfere with the desired visual effect. This is accomplished by checking the ‘Travel edge” box.