Often, atheists will claim that atheism is not a positive claim, but rather a lack of belief in gods. Such a position is merely negative, and doesn't tell us what the person actually thinks about the God question. This can be shown by looking at the possible positions one can take when one hears about gods:
1. Gods exist
2. Gods do not exist
3. I don't know
4. No one knows
5. I don't care
By saying that they "merely" lack belief, they are saying that they are not 1.
OK, that tells us what the atheist is not, but not what he is. By saying "not 1", the atheist still needs to clarify which one he is, 2 through 6. But if he would just tell us which one he is right off the bat, that would then entail not 1, and so there is no reason to start with not 1.