Signatures can definitely bring visitors to your site. It has nothing whatever to do with SEO though.
Apart from search engines, Sitepoint is one of the biggest referrer of those visiting my sites.
Most of those visitors will have clicked a signature link since the only time I ever post a link to my site within my post is on the rare occasion where the page directly relates to the question someone is asking.
SEO is never an end in itself. There's no point having the search engines list you as number one for a whole series of keywords if few people ever click the links in the search results and if those who do click back straight away because the page isn't what they are looking for. The desired end result is people visiting your site and then taking some action. SEO is just one way to try to get more people to visit.