Well, not that it matters, but someone told me that they were for certification. The flight deck was kind of dark during the day without them, so Boeing installed them to make it brighter in there.
Makes sense when you think of it. The forward and side windshields on aircraft with Eyebrow Windows (707,727,737,DC-8,DC-9,MD80) are really tiny. I could see how it would be hard to see the steam-gauge instruments during the day. With the advent of Glass Cockpits, seeing the instruments is not really a factor, unless it is in direct sunlight. So now you want less light, not more. Pretty funny if you ask me.
I think Boeing realized this on 717, finally designing them out cause we don't need them. I've never flown the 737, but I would guess that they are definetely more expensive and heavier than just a metal plug. I wouldn't be surprised if the FAA also requires them to be heated since thay are a window. Soemone then had the bright idea of removing them on the 73 line, which makes perfect sense. A cost and a weight savings.