[wdfifteen]

Initially wheels became larger to allow for bigger brakes. They became a fashion statement like hood scoops and shaker hoods did in the '70s and have been taken to ridiculous extremes. They are a huge market for wheel and tire manufacturers, so there is an incentive to keep the trend going.

edit: The brakes on that double naught DB in the OP look like the pretty much fill up the wheel, so maybe it isn't all fashion on that one.