You should do this because, in the event your email is hacked, your exposure will be limited. This may be controversial advice, in part because it’s cynical. That doesn’t make it any less important.
Remember the Panama Papers? Here, we are ignoring the substance of the leak itself to focus exclusively on what can happen if your email is hacked and there’s a lot of old and sensitive data to dig up. When considering your company’s data privacy and cybersecurity, it’s useful to imagine the worst case and take steps in advance to mitigate the potential fallout. Many businesses follow compliance rules related to electronic data retention. In those instances, or if hitting delete makes you queasy, consider moving email to an encrypted archive. Many don’t talk about this enough but it’s simple and free.