I think that if you post something that is to everyone's benefit and people recognize you as a knowledgeable person, they will click on your link to learn more about what you do.
Signatures are also good when you interact with others, they might want to learn more about you and check out the sites you have linked there. It's good to build trust with them and they may become potential great customers of yours.
We use our signatures to expose our events and free resources people. Free advertising for a free resources lets everyone win.
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I don't think signatures have any effect on SEO but are pretty good for traffic. I've noticed that a lot of traffic from signatures is from other hosts' curiosity, but that is ok too.
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Sean Shepard www.ethoshosting.com - Socially Responsible Managed Hosting
Sometimes I use my forum signatures to generate a little traffic for my websites, and sometimes I just add a little personal message. Depends on how I'm feeling, and what the purpose of the forum is.
I use my signature for branding and to let people know I can help with them. for example if they were reading a post and found it helpful they could contact me for further information on the topic, check out my brand, or know how I could help them with their venture. Albeit, my signature needs some work.
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Timothy Rhoads
Vice President of Sales and Marketing
Chihosting.com Hosting