They wanted to exploit a loophole in american politics. Before them, the other Native people who ceded their lands had first been recognized as nations and then when the territory became their legally, they would give it away in deals with the American government. The Cherokee thus declared themselves to be a sovereign nation and not to give the land, which is why they took the case to the supreme court in order to be protected.
Since southern seaports were closer to the West Indies (where British troops were stationed), the capture of Southern ports would allow their troops to move back and forth between the two regions.
They created more jobs to better the economy. These jobs mostly in manufacturing as that would be best for the war effort.
