The short answer is that if the trier of fact (i.e. an arbitration panel, or a Jury, or a Judge deciding a case without a Jury) that is responsible for deciding a case is unfamiliar with subject matter of the case sufficient enough to decide the case, then an expert witness is necessary to provide the information necessary for the trier of fact to be able to make their decision.
The slightly longer answer relies on the Rules of Evidence. An expert witness must have the knowledge, skill, experience, training, or education to help the trier of fact to understand the evidence or to determine the fact in issue. So again, if the fact in issue cannot be understood or determined by the trier of fact, then an expert witness is required to help the trier of fact to reach its conclusion. There are examples where expert witnesses can be required. Some motor vehicle accidents are not as clear as others, and an accident reconstruction expert is required. Some slip and fall cases require an expert witness to demonstrate how the particular surface that the victim fell on was defective. Most (if not all) personal injury cases require an expert medical witness to detail the victim's injuries and show the those injuries were caused by the accident in question. And, all medical malpractice cases will require an expert witness to render an opinion that the defendant deviated from the standard of care and that the deviation caused the victim's injuries and damages.
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