The main thing that censors were looking for in letters and postcards was information that would compromise what was called "operational secrecy." In other words, the censors did not want soldiers telling anything that would allow the enemy to get any information about what units were stationed where or what they might be planning to do.
As you can see in this link, soldiers were aware that their letters would be censored and they would not be able to tell such basic things as where they were stationed. If they enemy found out such information, they might be able to piece together what units were in what places and, thereby, what areas had strong defenses and which did not. If a soldier wrote "we are in (place X) and are about to move out" the enemy might be able to infer that an attack was planned somewhere near Place X.
Basically, the censors were just looking to delete anything that might be able to give a clue to the enemy about troop locations and movements.