There's a range of different circumstances in which a matter could engage the national interest. This is not an exhaustive list because, of course, facts, technology and the world change more quickly than we could potentially imagine today.
You can imagine a crime that may impact multiple provinces. You can imagine an opportunity to engage in international investigations that reach beyond Canada's borders. You can imagine circumstances where the severity of a given set of financial crimes could be funnelling into a criminal organization that would pose a public safety threat in many places in Canada.