Policy - Why are expert opinions needed?
In many cases, where generalizations are beyond everyday experience, a full, fair, and informed jury decision requires expert guidance to understand the facts. A dispute may involve info beyond the knowledge or capabilities of a typical juror, so parties are allowed to present testimony by expert witnesses.
But, because of the undue weight that jurors are likely give to exert testimony, rules are needed to limit the misuse of such evidence. Jurors are more likely to be "overawed" and accept the expert's opinions as truth, instead of using their own powers of deduction.