[kmontrose]

Don't forget user support. It's not all that uncommon for someone to forget their account, lose a password, or an email address. Circumstantial evidence can support ownership of the account, and let us fix things for them.

There are also errors on our end like account merge bugs, moderation mistakes, dropped/flagged/whatever recovery emails, and so on. Keeping additional historical data can help us recover in those cases.

If you're smart about what you track it's not that much data; we record most changes to user records into a history table (likewise, and for the same reasons on post records). Keeping traffic logs around and queryable forever would be really, really expensive though. We keep some around, but only really recent stuff is easy to query (about 2 days) since that tends to be what's needed when reproducing bugs. I don't even think we have all traffic history, and old stuff would require digging a tape out (if we even move those to tape like we do with DB backups, I honestly don't know; it's never come up).

Moderation is a good reason to keep lots of data around, you're right, but it's not the only one.

Disclaimer: Stack Exchange, Inc. employee.