Failure to preserve certain evidence in your case can be devastating. Depending on the circumstances, it could be the difference between winning and losing.
Take for instance a case where a chair breaks in a restaurant and you fall out of the chair and sustain an injury. That chair needs to be preserved, or at a very minimum several pictures of the chair must be taken. If the case necessitates an expert to determine whether the chair was faulty prior to you sitting in it, having the actual chair will be helpful. You will still need to prove several other elements of negligence of the chair owner, but having the chair is a good start.
The same can be said for auto accidents, especially if there is a dispute as to liability. For instance, if you are clearly not at fault for an accident, but the at-fault driver informs their insurance company that you are, the evidence can be helpful to make this determination. Having detailed photographs of the vehicles involved in the accident and the accident scene itself is vital. Depending on where the vehicles are located after impact can determine how the accident happened. If you ever need an accident reconstructionist later on, they will need this information. I have had many cases where just having photographs of the impact locations on a vehicle can get the insurance company to agree that based on the physical evidence their insured driver was clearly at fault.
Make sure that if you are ever involved in an accident to save any evidence you possibly can. You will need it later on down the road