A Russian sub in North Sea would prevent Germany from taking 4 IPCs of the Archangelsk convoy in R1! (And of course never again!)
(If Germany has “bad” rolls in attacking North sea maybe even as compensation to save the UK Battleship on occasion!)
Simoultaneaously it allows:
1. Agressive Russian play in Scandinavian territories (1 Art from the cash advance placed as Art in Karelia; transport can load 2 Inf from Archangelsk in R1) - to take IPCs and annihilate the isolated German troops there as fast as possible - and be back in time to face the main German threat! (More money, more inf-build possible for defending Moscow!)
2. Speed up UK retaking its convoy zones to allow for faster development.