it's because you can add a signature in forum. your signature can contain links your links can can help SEO
Good Forums have huge visitor base and the signature there brings in visitors along with link juice to you. Not all forums are useful and helps you with SEO/Traffic. You need to pick only those which are good
Forum posting helps to get updated about the new updates & techniques of the SEO. Also it helps to getting a backlink for your website from the signature
Quite funny to ask this question in a forum that people use for Link Building :-p But yes, forum can influence rankings / visibility, however, Signiture Links are not highly valued in Google. There are other ways to do this.
If forum allows dofollow signature then it can pass link juice to your website. And hence you can have top ranking in search engines and also you can get good PR for the website.
Forum posting is the process of commenting on or creating new discussions on forums to get backlinks. You get backlinks from the links at the end of your posts called signature links.
Signature links in forums are really useful to get a backlinks and it also help very little in grabbing traffic
Well mostly it's through the X amount of backlinks you have in your signature that count towards SEO.
2 ways: If the forum is related in topic to your product or service then you will win 2 fold, firstly for the link juice and secondly for the click through. If the forum doesn't relate directly to your product or service then you get the link juice still.
Well, why don't you learn how to research? I did a simple google search for "Forum posting and SEO", and came across this http://www.google-success.com/backlink-building-using-forums.htm
It offers some padding to your SEO strategy, but you can't rely only on forum signatures if you want to get some decent rankings.
first, if your site is related to the forum, it brings in direct traffic to your site and second, by adding links in forum replies and signature, you can increase your sites backlinks which is the most important part of SEO.