Maybe he's a thief who sells company business plans or customer lists. Perhaps she's the office intriguer trying to play people against you. Possibly he's a computer stalker like the fellow who shot actress Rebecca Schaffer to death. Conceivably she's a blackmailer. Maybe he's an old-fashioned voyeur.
Information is power.
Snoops want power. Whatsamatter, I've got nothing to hide. Show me an e-mail user who has no financial, sexual, social, political, or professional secrets to keep from his family, his neighbors, or his colleagues, and I'll show you someone who is either an extraordinary exhibitionist or an incredible dullard. Show me a corporation that has no trade secrets or confidential records, and I'll show you a business that is not very successful. Privacy, discretion, confidentiality, and prudence are hallmarks of civilization.