Companies tell employees to delete emails after 18 months because there are two alternatives: 1) delete emails after 18 months (or some similar time period), or

Companies tell employees to delete emails after 18 months because there are two alternatives: 1) delete emails after 18 months (or some similar time period), or 2) never delete emails, ever. If your policy is "delete emails based on personal preference", someone can sue you in court and claim that the selective deletions of email is evidence of wrongdoing even if you just happened to have a full hard drive that day. And you don't actually need to be a wrongdoer for someone to claim this.