Hitler very much didn't want a third front in the war. If Barbarossa had shifted to North Africa, it would mean fewer troops to deal with the soviets when they inevitably invaded (which Hitler was sure they would do). As it was, North Africa was (at first) an inconvenience to them. Mussolini had invaded North Africa to get his "empire", and when they started taking heavy losses there, it became clear that the Allies could take advantage of it to come up into Europe through Italy and the Mediterranean (which is ultimately what they did). Germany had to deploy Rommel and the Africa Corp. to help the Italians or face fighting a war on three fronts.

In short, Italy was supposed to be Hitler's protection against invasion from the south. But Mussolini was an idiot.

Edited to add: You must remember that at this point in time, North African oil reserves were for the most part undeveloped. As far as Hitler was concerned, it was just a bunch of sand.