Welcome to the forum, sfcdodge65.
The submarine’s surprise strike is it’s attacking shot in the combat sequence. So it does not get a second shot, when the other attacking units (if present) come to fire.
The advantage is obvious, when an enemy unit is sunk by it - in this case the defending unit has to be removed immediately and cannot fire back.
Battleships and Carriers are two hit units in Global. A single surprise strike hit just damages them. So in your given situation the outcome of the battle might be a lost sub on one side, and a damaged battleship and a surviving cruiser on the other side. So this is not a promising battle.
But think of other situations:
Regarding Carriers, the advantage of one hit can be huge: Planes coming from them are defending in the air - and they cannot land on a damaged carrier. So if they cannot reach a landing spot they will be destroyed. You have to think carefully before assigning a hit to a carrier. The carrier is reduced to a one hit unit until it will be repaired.
Regarding Battleships there is still an advantage left: The battleship, too, when damaged is reduced to a one hit unit until it will be repaired.
Or imagine two subs against one Battleship: With some luck both subs score a hit with their surprise strikes. Then the Battleship is sunk without firing back.