Does quality forum activity help you grow your business? Absolutely. Members get to know you, your level of knowledge and helpfulness, which all makes them more comfortable.
When a company posts their special deals in the advertising section, visitors and members appreciate knowing if the company takes an active role in the community. I shop the same way - if I see a deal, I research the company throughout and try to learn more about who is behind the account.
Strong relevant web hosting backlink(s) to your website certainly does help your SEO.
Whether a signature boosts that I am not sure. Regardless of that, we employ a nofollow script on signature spaces. From the rules:
If you are a spammer, DO NOT WASTE YOUR TIME. All of your posts will be deleted and account terminated.
If you are here to build signature backlinks (trust us, we know how to tell), your signature will be deleted in all of your posts and your profile page. NOTE: This is a "=NOFOLLOW" forum.
This is done to (a) increase the quality of signature content and (b) prevent signature spam, which helps both the community and account holder not get too many links in/out, which in some cases can do more harm.