Everything is now digital, and storage is cheap—why not save it all?
Sony illustrates the reason why not. The hackers published old e-mails from company executives that caused enormous public embarrassment to the company. They published old e-mails by employees that caused less-newsworthy personal embarrassment to those employees, and these messages are resulting in class-action lawsuits against the company. They published old documents. They published everything they got their hands on.
Saving data, especially e-mail and informal chats, is a liability.
The vast amount of data we generate daily can turn against us anytime in the future.