1.) Because assumptions are dangerous.
2.) So we know the degree to which they escalate tense situations, or otherwise make situations more dangerous.
One of the long-running memes regarding gun ownership is that "the only thing that can stop a bad guy with a gun is a good guy with a gun." This sentiment has been expressed by the pro-gun side every time there's a crime thwarted, and mocked by the pro-regulation side every time there's an accident. The truth is that we don't really know how much or how often the presence of a gun is going to cause a fight or a moment of carelessness or despair to go from bad to deadly, and on the other side, we don't know how often guns are used to protect people from harm. Studies have been done, but they've been flawed and/or limited in scope and both sides are guilty of cherry-picking data that suits their agendas. Having better data about how the presence of a gun influences other factors can help us better craft policies that prevent tragedies while also protecting freedoms.