[Conservababe]

Many people who are devoutly religious do not write the name God, but instead write G-d, because of their faith and reverence for the name.

Do not belittle them, please.

[College Repub]

It has to do with the belief that it's wrong to do things like write out the name of G-d and then erase it or tape over it (audio), or burn it if it's in written form, or erase it from a blackboard.