It is not because they want to see better. It is because they want TO BE SEEN in foggy or heavy rain condition.

If I have to stop suddenly where visibility is limited, I instingtively will reach for the hazzard just to increase the possibility that the driver behind will see me until I move forward again..

BTW it is widely practiced in several Asian countries ,mine included, so it is not exactly monkey see monkey do (unless you want to imply something....)

cheers
SK