He wanted Russia's wheat and oil fields.
By 1944, Germany war machine was in tremendous need of oil, and could only supply around a quarter of its own needs, the war on the Soviet Union intended to reach the rich Oil fields in the Caucasus, which would also allow them to control the famous "mitterland".
The seizing of the Mitterland (or the land of the middle in reference to the Eurasian continent) would be a key geostrategic point. Since the war effort had caused a huge demand in energetics, Azerbaijan Baku's oilfields were the target to the Germans and strategic as well due to the USSR's heavy reliance on Caucasus oil.
Hitler was eager to capture Baku, securing oil provisions for the rest of the late stages of the conflict and also solving the absolute necessity of occupying the huge oil region of the Caucasus. As Hitler reached oil fields in the region, he found the Russians had trashed them intentionally and set fire to them, rendering them useless. Then the Red Army stroke back at Stalingrad where they resisted and under a "no turning back policy", defending the motherland at all costs and this fiercly began to expel the Nazis back and were never able to recover from that major strike.
