Why did the south have to keep their ports open during the civil war?

The South didn't have much industry, it was mainly a farming area. Their main export was cotton, I believe another top export was tobacco. The South didn't farm very much food, and with little industry they didn't have basic supplies like shoes. So the South needed to keep their ports open so they could receive food, shoes, and other important supplies.

As a side note: The North did a pretty good job of blocking off Southern ports so the South experienced much hardship during the war.