Yes, I believe Mississippi and Alabama were very important in the war effort, all through the war. Especially, since they controlled rivers into the interior from the Gulf of Mexico. With the Union forces in control of those rivers, Georgia would have been flanked on the west and from the east by the Atlantic.

Northeastern Mississippi was the bread basket for food production for a large part of the South.
Selma, Alabama was a major military depot, foundry, and ship yard.
Mobile, Alabama was a major hub, port, and host to many factories.
South Alabama was a major producer of salt for the South.
Meridian, Mississippi a major hub for railroads, especially the GM&O line.

With those states gone, it would have been difficult for the rest of the Confederacy to stand for long.

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David Upton