A personal injury trial can require knowledge of much more than just personal injury laws. A lawyer often needs to present argument and testimony that go beyond law and into varying fields of expertise.
A medical malpractice case will require argument about medicine and anatomy. Negligent security cases may require testimony about generally accepted security procedures. Slip and falls may require argument about building codes. Any case involving a product may require knowledge of how the product is assembled. These are all areas that are often outside the technical expertise of both lawyers and juries.