Keeping Russia supplied could increase German pressure to risk a fleet action in search of a breakout. Submarines can deny an enemy the use of the sea and help maintain control over it. The benefits of that control still depend on moving troops, goods, and supplies. Surface shipping performs those tasks, so exploiting an open maritime route requires surface forces.
Notable Point · 1:16:51-1:17:50
Why sea control still needs surface shipping
Submarines can deny passage and help maintain sea control, but exploiting a route still requires surface shipping to carry troops and supplies.