Why did navies experiment with cruiser submarines?

What was the logic behind several navies experimenting with cruiser submarines?

Gun ammunition allowed many more attacks on merchant ships, leaving the submarine's limited torpedoes available for enemy warships.

The attraction of a cruiser submarine began with magazine capacity. Torpedoes were large and could be carried only in limited numbers, while a submarine could stow many more gun rounds. Designers therefore imagined using gunfire against merchant shipping and reserving torpedoes for enemy warships or especially valuable targets. This logistical logic explains why several navies tested the concept even though the resulting boats were poor surface combatants.