It's good to clean, I think after a while so it does not get filled with logs of what members and what admin does. From a security perspective, if my admin account is hacked, I do not want the hacker to see any logs of what I have done or what members have done. It may be good that logs are saved for some but for me it is an unnecessary thing. It's like discussing whether white is a nicer color than black. There is almost no answer to that but what you want yourself.
On phpbb, it was just a matter of pressing purge and it was done.