What seems like a lifetime ago I worked for a few enterprises doing various things like firewall configurations, email system optimizations and hardening of Netware, NT4, AIX and HPUX servers. There were 3 good sized employers, a bank and two huge insurance companies that both had financial components. While working at each and every one of them, I was, subject to their security policy (one of which I helped to craft, but that is a different path all together), and none of which really addressed data retention. When I left those employers, they archived my home directories, remaining email boxes and whatever other artifacts I left behind. None of this was really an issue for me as I never brought any personal or sensitive data in and everything I generated on site was theirs by the nature of what it was. What did not occur to me then, though, was that this was essentially a digital trail of breadcrumbs that could exist indefinitely. What else was left behind and was it also archived? Mind you, this was in the 1990s and network monitoring was fairly clunky, especially at scale, so the likely answer to this question is "nothing", but I assert that the answer to that question has changed significantly in this day and age.
Liability is a hard pill for businesses to swallow. Covering bases is key and that is where data retention is a double edged sword. Thinking like I am playing a lawyer on TV, keeping data on hand is useful for forensic analysis of potentially catastrophic data breaches, but it can also be a liability in that it can prove culpability in employee misbehavior on corporate time, resources and behalf. Is it worth it?
Since that time oh so long ago I have found that the benefit has far outweighed the risk in retaining the information, especially traffic data such proxy, firewall, and network flows. The real issues I have, as noted in previous posts, is the correlation of said data and, more often than not, the archival method of what can amount to massive amounts of disk space.
If I can offer one nugget of advice, learned through years of having to decide what goes, what stays and for how long, it is this: Buy the disks. Procure the tape systems, do what you need to do to keep as much of the data as you can get away with because once it is gone it is highly unlikely that you can ever get it back.