Tragedy Example 1: Celebrity drug deaths. Michael Jackson died of an overdose of Propofol. If he didn't die that way, either we wouldn't be as aware of Propofol abuse as we are now, or some other famous person would have died in the same way. Erasing that tragedy from the timeline could have just postponed Jackson's untimely death since he already was an addict. Jackson's death also helped alert us to the tendency of some doctors to be enablers in their patients' addiction, contrary to the Hippocratic oath.
Tragedy Example 2: Plane Crashes. The fatal plane crashes caused by malfunctions alert us to existing problems in plane design and help future engineers solve these problems.