For the Gen III the daylight sensor is used in the Climate Control system. If you have Auto Headlights it is used for this feature. Also it is said that it is used with the instrument panel lights if you go through tunnels, etc.
It looks like something aftermarket to me. Never seen anything like that on Prius before. Could be a satellite radio antenna. - Alex
That looks like the mini-dash mount Sirius Satellite radio antenna Ive seen for sale at ecomodder or some other green site.