Occasionally staring at the amount of stuff in my email inbox and archives I wonder the same thing:
"Why not read an email and then instantly delete it? Why do we save emails? Why do we archive them in folders for safe keeping? We don’t save phone calls. We have a conversation on the phone and then we hang up. If we need to take notes for whatever reason we do, but 99% of phone calls are completely ephemeral. And if we forget something, or we need it again, we just make another call.
Is email really any different? Are we all keeping emails around just because we can? Do we really need to have this stuff on hand so we can go back 14 months from now and dig something up? If we need to dig something up why don’t we just ask the people who we were talking to originally?"
My rule for my inbox (either email or physical, old fashioned- dump-it-in-Shannon's-tray type inbox) is pretty simple. Action it as quickly as possible get it out of there, and get on with your day. I do the same with notes that I take over the phone. I either action it straight away, or stick in in my inbox to be action later that day.
Of course it doesn't always work like that. I occasionally keep some emails, but that is a bit like keeping that old bit of junk in your garage, "just in case you need it." Most times you never need it again, and if you do, it needs to be updated anyway.
I guess keeping emails can be like security blanket- it's easier to sleep at night if you know you have kept everything, just in case. If I do that, it's usually about a week later that I look at it wonder why the hell I kept it, and hit delete. All it does is delay the decision making.
As for archives, I can't be bothered with archiving stuff. If it is important, I print it and file it it physically. At least then I don't waste time going through old emails trawling for stuff that I can probably go back to the source and find out more quickly.