It seems like users will tend to add this tag whenever they have some feeling about the "bigness" of their data. Of course that doesn't mean the tag is useful or relevant.
So, when is this tag useful and relevant? It's already difficult to say what "big data" even means, as discussed in one of the early and most popular questions on the site. Put another way, when someone views all questions tagged big-data, what sorts of questions should they see?