Why would the Union forces burn down the plantations in "Gone with the Wind"?

Margaret Mitchell's work displays the rather horrific
effects of total war, the policy advocated by Grant and Sherman towards the South in the
closing stages of the Civil War.  This involved showing little mercy to the enemy and
sought to make a statement about the South's desire to secede in the first place. 
Sherman understood it as involving a "scorched earth" policy where all of the South
would burn.  This included plantations as well as the city of Atlanta.  The burning down
of the plantations was a symbolic act, and Mitchell does not miss a chance to depict it
as such.  The burning of the plantations represented the ultimate blow to Southern
pride, and also helped to reduce the South to a position where unconditional surrender
was the only option.  Mitchell's work shows the Union forces burning down the
plantations to reflect a "death" of the Southern way of life.  The condition that
Scarlett used to live in and used to believe in is now literally in flames, causing her
to redefine herself just as the South was forced to redefine itself.  This Union forces
burning down plantations in the South was an integral part of this
redefinition.