The confederates burned Richmond for strategic reasons:
1) The Union cannot use Richmond as a staging area. If Richmond became a staging area, it would have not only a moral effect on the Confederate troops, but also allow the Union to regroup and press the attack.
2) There were a large amount of resources that could not be transported away from Richmond on time. However, if the resources fell into the hands of the Union, it would have spelled a quicker defeat for the Confederates.
~Rise Above the Ordinary, Senpai
