Rostov and Rostov alone, because it is a MAJOR choke point against the fascist forces, if you want to keep them out of your capital, AND out of the oil fields.
Rostov, is a VERY likely position for the bulk of your army to sit, if you are facing a significant german invasion from the south. Any movements north, and you’ll counter attack - regrouping into your latest equipment build into your capital with a retreat. Any Movement to the south, and they’ve got to go THROUGH you. It also puts you in position to LIBERATE the caucusus, and killing Germans piece meal on YOUR OWN terms, is always a better idea.
Lastly, In short It’s ROSTOV because you want your planes to be with the rest of your army.