It is a matter of reward, threat and counter.
Keeping Germany to Karelia in G1 means that Russia has more income, controls the center of Europe, and can attack in maximum force in ANY territory Germany moves into.
Allowing that ARM to Archangel means that Russia HAS to send forces to Archangel in R2… unless they want to leave an ARM on Moscow’s doorstep and surrender 2 IPC’s to Germany… That means forces devoted to Archangel instead of to Belo, Karelia or Ukraine. And that force diversion means greater losses for Russia in those other battles since overwhelming force not only increases the odds of winning, it decreases the number of lost units by negating second or third round defensive fire. THAT is a significant edge for Germany… weakening a Russian attack at the cost of only 1 ARM, which paid for itself with 4 IPC of territory the previous turn and the 50/50 of killing an enemy unit on the way out.
Also, I am not saying “stage in Karelia” and “Attack Caucuses” as being simultaneous options. They are options and Russia leaving the north vacant after R1 gives Germany both options… for free. However… if Germany CAN sucker Russia north, then Germany can hit in the south with up to 4 INF, 4 ARM, a BB shot and their AF WHILE STILL HITTING EITHER WR OR ARCHANGEL WITH THE KARELIA STACK.
Massed land forces are perhaps the most powerful offensive and defensive move in the game. Making Russia split their forces 3 ways instead of 2 seriously weakens Russia and allows Germany to take out units with minimal losses. Maximum forces in WR is the best deterent that Russia has. They block the central avenue of attack and can immediately counter with all INF, ART and ARM into Archangel, Karelia, Belo, or Ukraine. Split up to any degree, they take more losses on either attack or defense… and split into Archangel removes INF screen from attacks in Belo, Ukraine, or Caucuses.
Germany loses 1 tank but gets 4 extra IPC’s. Russia either accepts that economic loss, or splits their forces to some degree and allows Germany to destroy those forces for far less cost than would be possible if they remained stacked. And Germany can do so without breaking their own stack, making Russia’s losses even higher on THEIR counter.
Block in Karelia. Lose 1 INF, maybe take 1 with you. Make Germany reach Archangel on G2 instead… after the UK has built up and staged their Navy for a counter-attack… after Russia has built more forces to swamp futher advances… and with the economic shift being 4 better for Russia by only being down Karelia instead of Karelia AND Archangel.