Many cultures have traditions that, over time, have simply become something that "people do." The same can be said of Judaism and the Jewish custom of spelling the word "God" as "G-d" in English.
In Jewish law the name of God is treated with a great deal of respect and reverence. When the name of God is written or printed in Hebrew, that text cannot be erased or destroyed. This is why siddurim (prayer books) are ceremoniously buried when they are no longer being used - the books cannot be destroyed since they contain the name of God.
Though there is no prohibition against erasing the word "God" in English since it is not one of the Hebrew names for God, many Jews have nevertheless taken to replacing the middle "o" with a dash or exclamation point. When written this way, they feel better about erasing the word "God."
You can learn more about this custom and about some of the Hebrew names for God in: Why Do Some Jews Spell "God" G-d?
Image credit: Getty Images/Ed Honowitz