Everything in a guest account is deleted on logout, so there are no logs to manually remove. I'm not sure where the logs are kept for a managed user account but they cannot be deleted through the Parental Controls interface. The logs allow the administrator to see what websites were accessed by the managed user, what sites were blocked by the content filter, the applications used by the managed account, and also any iChat chats which were initiated. The administrator can then make decisions about restricting or loosening the existing limitations.