Forums are used to discuss queries, problems and issues. Forums are good source to get quality back links as well as reputation and to generate traffic. Through signatures, you can get back links. In forums, you need to keep participating in threads.
Active participation is very necessary in forums otherwise your account is closed. You can’t use forums only for signature linking. Starting new threads, participation in threads and to reply of threads are necessary to get advantage of back links taken from forums.
Forum participation is offpage optimization technique.