One bizarre trend emerging in this new RWD era is desktop-browsing web designers resizing their browsers to watch the break-points change the layout. The average user never sits at their desk repetitively shrinking and expanding the browser window like a mad scientist. Yup, we're all strange.
Using a graphic where the solid parts match the background of a site and the transparent parts make the shape, we can let a background color bleed through. This allows us to set that color through CSS, and do any fancy thing CSS can do, like say animate that color.