I often see Photoshop files where the background is filled with a flat colour. The simple way to do this is to simply select all and fill with the desired colour – but what if you change your canvas size… back to step one because your left with a rectangle filled with one colour surrounded by your background colour! There is a simpler, more flexible way of-course!
Sometimes you will want to make a marquee selection, and move it whilst still drawing it (you still with me!). So rather than draw it and then move it, this technique allows you to draw it AND move it AND THEN still keep resizing your selection.