In the entirety of a person's life, having one person in your life can make all the difference (either positive or negative).
For example, What if Charles Manson never met Tex Watson?
If Tex was never involved with Manson's group, Lotsapoppa doesn't call the ranch looking for Watson (TJ Walleman answered in OTL. He gave the phone to Manson when Crowe asked for Charles because he knew Watson as Tex).
As a result, Manson doesn't go to Bernard Crowe's to rescue Watson's girlfriend, and he doesn't shoot Crowe. That started the chain of events that ended with Tate, Sebring, LaBianca, etc.. getting killed.
Without that killing, Manson doesn't turn off the record producers (Melcher, etc...). They put out the Manson record on Capitol Records, and they do a documentary about the Spahn Ranch.