Watch towers don't seem to do anything at all.

I built a watch tower at every single outpost plus main city.  I didn't really notice a difference so I deconstructed a watch tower at one of my outposts.  Then I advanced one turn.  No difference in vision range.  I rebuilt the watch tower again and after it completed there was no change in vision range.  I then advanced one turn just in case it was one of those things that doesn't become available until the next turn.  No change in vision range still.