Basically, it is very important, in terms of site security, to keep good logs of your web server.
Proper logs can provide sufficient proof of a user's activities by logging the actions of that user.
For the most part, this is usually unnecessary unless there has been some questionable activity on the web server. Proper logging provides a means of non-repudiation for security officials in the event that someone has acted maliciously against your web server. It is important to note, however, that there are certain guidelines on gathering and retaining information. The CIO web site points to item 13A for guidance on the retention of web logs. Item 13A is found on page 2-16 of the NASA Records Retention Schedule PDF file.