Evidence is the crux of any legal case. The truth, of course, is of paramount importance, but what is also key is what can be proven. Without the right amount and type of evidence, a case will go no where.
This is as true for personal injury cases as it is for those involving crimes.
Because certain evidence can be considered critical to a case, and those with vested interest in not having that evidence presented may be in possession of it for a time. The courts have long recognized the importance of sanctions for loss or destruction of evidence - regardless of whether it was intentional. When important evidence is lost, it's called spoliation. By imposing sanctions for spoliation of evidence, the court provides incentive to protect key evidence that may be helpful to the opposing side.
Sanctions can include anything from a special instruction to the jury to a default judgment on the issue of liability or damages. It can result in the party disadvantaged by the loss of evidence winning the case.