What is that putty on windshield pinch weld

The rubber gasket only serves as a medium or filler between 2 less that perfect planes, the fence, and the glass. A cross section of the gasket looks like 'H' The edge of the glass is a lot closer to perfect but the fence is made up of several pieces pinch welded together. I personally have spent a little time tweaking the fence for a better result.
If the raw edge of the glass stands in water the glass will deteriorate. This causes the milky discoloration seen in old auto glasses.
So... the more butyl sealant you load on the more mess there is to clean up and the less chance of leak.