Why was it important for the Confederacy top defend Richmond, Virginia?

It was important for the Confederacy to defend Richmond, Virginia because Richmond was the seat of the Confederate government (its capital). The loss of Richmond would be the Union equivalent of losing Washington D.C. to the Confederates. It was widely considered that the capture of the other side's capital would signal the end of the Civil War.

the us constitution is a living document, meaning that it can be changed over time as the country changes.

a case involving the laws of a single state.