My tax returns are emailed to me. Even though I have never needed them I'm legally required to save them for 7 years just in case. I have a number of other emails in the old list that I don't expect to ever need again but they are still relevant to something and I need to keep them just in case. Odds are my house will not burn down, but I still have fire insurance - most of my old emails are like that: I don't believe I will ever need them but I can't prove it.

At work I've discovered outlook has an expires after tag that I can set on each email - I set this on everything I save which keeps my saved messages clean. I haven't found a convent way to do that with anything else though I understand something like it exists.