Can someone explain how private emails are used in prosecutions? If it's a blackmail or intimidation then a victim can supply all the necessary evidence. If it'

Can someone explain how private emails are used in prosecutions? If it's a blackmail or intimidation then a victim can supply all the necessary evidence. If it's an illegal trade then there should be traces in the real world. In what situations private emails of criminals are absolutely necessary? The only thing I can imagine is an exchange between two criminals of classified information for virtual currency.