Zeppelins were primarily used for bombing runs during World War I. Though they had mixed results at this task, they were also effective in intimidating enemy forces and in transporting men and munitions.
The zeppelin was the first vehicle to make targeted bombing possible. It was able to fly higher and farther than conventional aircraft of the time and could carry more cargo. Zeppelins presented an easy target for artillery, however, so they primarily flew at night, which made bombing difficult due to visibility. This limited their effectiveness at the task.
Both the Allied forces and the Germans used zeppelins during the war, but they were more commonly used by the Germans. The zeppelin enabled the Germans to cross the English channel and attack civilian targets, which they believed would erode public support for the war. Zeppelins were very effective as a psychological menace, but British cities were able to implement measures to limit damage from their often inaccurate bombardments, and they didn't have the desired effect of removing Britain from the war.
Some other uses for zeppelins were to spot submarines, coordinate troop movements by radio, and carry both troops and cargo. Both the Allied and German forces used them for these purposes.