Aaron hits on an interesting point pretty much in passing: Gmail's UI is pretty encouraging to the archiver vs. the deleter. The Gmail Android app, for example, has a "swipe to archive" feature that lets me push the message back to my archive with a single motion. If there were "swipe to delete," I'd probably end up deleting a lot more.
The real reason I save so many messages, though, is honestly because it feels safer to do so: It costs me essentially $0 to archive a message instead of delete it, and if 10 years from now, there's a little transactional email that helps me win a lawsuit or something, I'd be glad to have it. If I paid for Gmail storage like I paid for storage elsewhere, I'd probably think a little harder about the value.
The real reason I save so many messages, though, is honestly because it feels safer to do so: It costs me essentially $0 to archive a message instead of delete it, and if 10 years from now, there's a little transactional email that helps me win a lawsuit or something, I'd be glad to have it. If I paid for Gmail storage like I paid for storage elsewhere, I'd probably think a little harder about the value.