Backstitch is used to define the area between the diagonal stitches.
Using two colors makes a nice overall look for this stitch.
Using the backstitch to cover the area between a block such as the Algerian Eye stitch where there may be some white from the canvas showing.
The backstitch can be used with various types of stitches besides being used to outline an area on the canvas. In the sample below, the circle was backstitched to outline it and show it off a little bit.
Using a backstitch between the Satin stitches at the top of this sample gives the area a definite depth that will look like a ridged area.
While not every area on your canvas needs to be backstitched, when you are planning your stitches make sure to add this stitch.