You are not able to post new articles or reply to existing articles while browsing the forums without logging in.
Other advantages you would get by logging in are
The server records which articles you have read so next time you view a forum you can easily tell which articles you have read before
You can choose to ignore threads which means that you won't see any articles in that thread in future
You can choose to watch selected threads and at later times choose to only show watched threads in the forum article list
For forums that have enabled it you can give a rating to each article and the average rating of each article is displayed to all users, who can optionally filter out articles with low ratings
You can search all articles in all forums for particular words
Automatic adjusting of article posting times to your time zone
Choose to only show threads containing unread articles in the forum article list