OK, I know that they were for protection - you closed up the house when you went away, or against a coming storm. Or they kept out the cold - 'Indian shutters', which came about after the Revolution, were not to protect against Indians, but for warmth in winter, 'coolth' in the summer, and for visual privacy from the street. And I know that our ancestors closed in their houses because they considered 'night air' dangerous.
I also know that 1) most houses were designed with windows directly opposite each other to encourage air flow, 2) in climates warmer than the north east, the kitchen was completely open in the summer - hence the 'dutch' door, where the bottom part keep animals out and small children in.
But as soon as the circular saw allowed us to make fins, shutters became popular in a way that they hadn't been before - they become standard equipment. And then it's not until the 1920's that they become decorative, with little cut-outs on the upper panels.
Why? Are the reasons I've listed enough? I have read that the shutters protected fabric from fading, that a dark interior was fashionable. I am still skeptical, I think these are secondary benefits. The Park-McCullough House has movable interior shutters on all windows, even in the servants' wing. It also has, as original equipment, louvered doors as second doors from the bedrooms in the main house into the upstairs hall (which is a room about 14' wide and 50' long). The House also has a belvedere, (a tower in the center of the house for surveying the beautiful countryside) which even today in the summer easily cools the House. The scientific principal behind it is called a Venturi.
I was recently reminded that mosquitoes do not bite when there is a breeze. I also know that the technology to make windows screens that could protect against mosquitoes was not really available until the 1890's. ( It has to do with weaving fine wire mesh .)
Then I thought about how in the evening the heat of the day could escape out the top of the house, bringing the cool air in from below, and keep the mosquitoes moving. And provide privacy...
I think I've got it!
Victorian Interior Decoration, Winkler and Moss, Henry Holt, NY,1986
How Night Air Became Good Air,1776-1930, Baldwin,
Environmental History, Vol. 8, Issue 3
and conversations with John Crosby Freeman, "The Color Doctor"
Here is the whole series:
Part 1 - http://www.jgrarchitect.com/2008/04/building-to-weather.html
Part 2 - How does the carriage house work with the sun to minimize wind chill?
Part 3 - Why bother with a cupola?
Part 4 - Eaves? they're important?
Part 5 - How a floor plan makes a difference:
Part 6 - A look at how these concepts were used at the Big House:
Part 7 - Shutters: