Actually, it's almost certainly as the other person stated - for administrative moderation purposes. There is no other purpose to maintaining historical backups of this sort of data. Especially not when that costs money.
When I built a site that existed for a very long time, was very popular, and involved monetary transactions, I had to track nearly everything. IP addresses, address changes, email changes. Everything I could think of. This was then utilized when I suspected someone of fraudulent behavior. I could pull up an administrative screen that compared data in an archive copy (where I dumped the older information for just this purpose and to specifically keep it inaccessible to the outside world for user security purposes). With that, I could see whether several users were actually the SAME user. I even tracked things like user-agent string and detected screen resolution.
A lot of pieces of data can come together to provide more than circumstantial evidence that someone is shilling, trying to feedback-bomb another user, and so on. Enough correlated points of data can confirm suspicions like this. You'd be surprised how many people use an email address for one account, change that address, then create a second account with the email address they used to have on the first account and then use the second address to drive up the value of their stuff by shill-bidding against another user on their own item.
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.
