When you start with an image like a jpeg, it is a "background layer". If you use the eraser tool, it will leave whatever color you have selected as the background color in your palette wherever you erase. When you promote the background layer and turn it into a raster layer, if you erase, it leaves transparency wherever you erase. There are MANY cases and situations where that is much more desirable then leaving a "color" behind when you erase, especially if you have added any layers "below" the original layer.