Why was blockading southern ports and gaining control of the Mississippi important for the North?

The south had an agricultural economy. The south grew cash crops like cotton and sugar. These crops were then exported and sold overseas. Then the cash was used to import the materials that the south needed.

During the Civil War the south needed military supplies that the o south could not produce itself due to the lack of manufacturing in the south. Cannons, rifles, ammunition, and medical supplies had to be imported from Europe, or captured in battle from the North.

The blockading and capture of southern ports hindered or prevented the importation of these critical war materials. Also the blockade prevented the Confederacy from exporting the cotton and sugar that was needed to pay for the military supplies.

After the blockade was firmly in place, the Confederacy could only trade by transporting materials across the Mississippi River through Texas and Mexico. The fall of Vicksburg on July 4th 1863 denied the Confederacy the use of the Mississippi River cutting the south off from trade with the rest of the world.

Without a sources of imported military supplies the south was at a disastrous disadvantage in decisive military engagements. This was aggravated by the fall of Atlanta one of the few manufacturing center of the Confederacy.