Why did the Confederates defend Petersburg so fiercely?

It was the new Confederate capital.

It was a rail link to the Confederate capital.

It was a troop center for reinforcements.

It was in a northern state.

It was strategically important as a key supply line for the Confederate army, connecting Richmond to the rest of the Confederacy. Losing control of Petersburg would have severely compromised the Confederacy's ability to sustain their war effort.