Why did union forces want to take control of mississippi river and confederacy in two?

Union forces wanted control not only of the Mississippi River, but the Ohio, Tennessee, and Cumberland Rivers (which they achieved by capturing Ft. Henry and Ft. Donelson). The Mississippi gave them a water route to use for movement of troops and supplies into the South, and by following the River to New Orleans and capturing the city, they were able to close off a major port of the Confederacy. (Control of the other rivers also provided a route to penetrate farther into the south with the assurance of the river to facilitate supply lines -- without that necessity, they were significantly limited in their ability to advance, needing proximity to Union-controlled territory or a railroad, and railroads in the South were few.)