Oftentimes the records aren't in the hands of either party and need to be subpoenaed. When you get them, they can open up entirely new lines of inquiry. Opposition will fight this tooth and nail so that the evidence can't be included, or they'll go on a fishing expedition under the guise of having all the facts on the table, and the court might just allow them. This process can take a very long time, and from what I've seen, the higher the stakes, the more the court will be willing to allow it to happen, so nobody can cry to the appeals court that something important was left out. Judges don't like their rulings overturned.