In the world of modern trials, expert witnesses are the coin of the realm. Lawyers know that most of the time, experts are case-breakers. Their demeanor, knowledge, and presentation ability are key qualities. Accordingly, their persuasive effect on modern lay jurors makes it incumbent on judges to ensure that an expert's opinions are appropriately directed. That means not allowing an economist to testify about the medical dynamics of bone disease, for example.
Benner, Brian and Carlson, Ronald L., "Gatekeeping after Gilbert: How Lawyers Should Address the Court's New Emphasis" (2006). Popular Media. 58.