In Eastern front terms, the German Afrika Korps would've been nothing more than a drop in the ocean, and far less strategically useful than they were in Africa. The actual German component of the DAK was only 2-3 divisions. At the same time, they were fielding between 150-180 divisions plus dozens of allied ones in Russia. The entire Afrika Korps would've struggled to make a difference in any particular battle in the East, much less tip the entire balance of power there. Also, logistics was an issue. The German Army always struggled to properly supply their troops in the vastness of Russia, and it was supply, rather than the availability of troops, that was the limiting factor for Axis forces on the Eastern Front. Even if they had re-deployed those forces east, they would've likely sat there starving without fuel or ammunition. Now compare that to what they accomplished in Africa, where 2-3 well-handled divisions were sufficient to put the fear of God into the British and threaten Egypt several times. In terms of "return on investment", Africa was probably one of the best options for those troops.

It isn't until the Battle of Tunisia, after the Battle of El Alamein and Invasion of North Africa, that your question really applies. At that point, the question of who was winning Africa had really been answered. The decision by the Axis to pour hundreds of thousands of troops into Tunisia (including about 8 German divisions), where they accomplished little besides getting captured, is harder to understand and arguably did waste troops the Axis could've used more profitably to defend Italy and recover from the Stalingrad disaster. That deployment is hard to justify, for exactly the reasons your question suggests.

In general the Germans didn’t have all that many men in Africa. The bulk of the fighting in Africa was performed by the Italians. Just to show a few snapshot moments:

• August 1941 - With Barbarossa was full roar, there were 149 German divisions in the USSR and Finland. There were 2 in Africa. In comparison there were 4 in Germany, 40 in “the West” (Low Countries and France), 7 in Norway, and 7 in the Balkans.
• January 1942 - With the Germans falling back in front of Moscow, there were 160 facing the Soviets. There were 3 in Africa, 36 in the West, and 7 in Norway.
• August 1942 - There were 186 divisions against the Soviets, 4 in Africa, 36 in the West, and 7 in Norway.
• January 1943 - During the battle of Stalingrad, there were 198 divisions against the Soviets, 7 in Africa, 48 in the West, and 12 in Norway.
• May 1943 - This was the height of fighting in Africa, the Tunisian campaign. There were 192 divisions against the Soviets, and 11 divisions in Africa and Italy combined. There were 13 in Norway, and 56 in the West.

At all times there were more Germans in Norway on garrison duties than there ever were in Africa. Likewise the Balkans usually had around 50% more divisions than Africa. And the West always had far more units. Now that’s not an entirely apples to oranges comparison. Garrison units consume far less supplies, and mostly were static units assigned smaller numbers of most types of equipment. And the Afrika Korps/Panzerarmee Afrika had a high proportion of tanks and trucks. The fact remains that the Germans could have generally gotten more men by simply cutting the Norwegian occupation force in half, than they could have by entirely eliminating the Afrika Korps.

With that said, to answer your direct question, by fighting in Africa they were keeping the Italians in the war. Withdrawing from Africa removes a lot of the mobile forces from Africa and the British had proven they could beat the Italian infantry if they didn’t have an adequate armor force supporting them. Without the German contribution, there’s a decent chance that the British advance a year earlier. If a handful of divisions could keep the British out of Italy, it was probably a fair price. This is especially true as the Italians sent 10 divisions to the Eastern front, the Italian 8th army. Supporting Italy with (generally) 3-4 divisions seems like a fair exchange.

I do think the Tunisian campaign is where that argument breaks, though. Once driven to Tunisia and caught between the Americans and British, the Axis had little hope. Rather than withdrawing and making their defense in Sicily and Italy, Hitler reinforced the surrounded pocket in Tunisia, ensuring as many Germans as possible would be killed or captured. Rommel with 1-4 divisions keeping the British occupied in 1941 and 1942 in the Libyan/Egyptian desert was a great use of resources. Rommel getting 9 divisions captured in Tunisia was a terrible blunder.

In strategic terms, the North Africa campaign was a battle for the Mediterranean. Germany in mid 1940 had knocked out France, and and that point that left the UK as the sole serious threat to Axis domination of the Mediterranean. With the relentless bombing campaign upon the UK itself and Germany pouring a huge amount of resources into submarine warfare, the Axis hoped that this left the British garrisons in Egypt and North Africa as a whole vulnerable. If the Axis could seize Egypt and the Suez in the east, and eventually Gibraltar in the West, they would have complete domination over the Mediterranean. This would have given a huge resource boost to North African and Middle Eastern resource access to the Axis, and hampered Allied logistics immensely.

Unfortunately for the Axis the Italian-German invasion of Egypt failed, and the next three years bogged down into the eventual collapse in the Battle of Tunisia. Soon after this collapse in May 1943,the importance control of the Mediterranean became self explanatory as the Allied forces brought the war directly to Germany's biggest Ally in July with the invasion of Sicily and by September 1943 they were on the mainland of Italy.

Arguably, if the Germans hadn't tried to go on the offensive in 1940 and then put up a serious defensive effort in North Africa, the Allies would have exploited the situation by expelling the Italians from Libya and then landing in Italy sooner than they did.