In world war i, both submarines and zeppelins allowed combatants to  bomb enemy trenches  deliver large numbers of troops  reduce marine traffic  spy on the enemy

Spy on the enemy since submarines couldn't possible bomb enemy trenches or deliver a large number of troops. Although a zeppelin could bomb a few ships, they could also be used to spy on the enemy just like a submarine. I'm fairly certain they allowed combatants to spy on the enemy although I could be wrong and they were used to reduce marine traffic.