The simulation uses submarines as a local denial force around home ports. Their presence makes attacks and blockades more costly for the enemy. That pressure reduces the need to hold expensive surface units back for static defence. A comparatively cheap local force can therefore free the main fleet to concentrate on offensive operations elsewhere.
Notable Point · 2:35:39-2:36:03
Submarines can free the battle fleet from port defense
In the game's operational model, cheap submarines make attacks and blockades around home ports costly enough that the main fleet can concentrate on offensive operations elsewhere.