It is a big part because people advertise their sites, and many other things. I do it at other sites as well.... I am on at least 15 different forums, and each forum I have an advertisement for this site. the MAJORITY of the people are advertising something. Whether it be their playasia.com deals.... their sites, sites they visit.... or other random stupid crap in sigs that just have no sense. (and yes, I still post at these other sites when I get time).
We've had sigs for quite a long time at our old site... and I've seen them enough in my time of visiting other ... I know what they are about.
75% of the Ezboard sigs were advertisements. AT LEAST 50-60% on the boards I've been to are advertisements.... for sites, what they are selling in the selling forums..etc.
Sigs are a dead topic here... we've seen a huge improvement when we killed them off a few years ago, and we are not going back. If people cannot enjoy a site because they do not have sigs, then maybe they are here for the wrong reason. we have great members, nice topics... and that is the main important ingredient.
there is a chance I'll add something else in the future... but I am not sure. It is like a single page in everyone's profile that they can edit with their information. BUT then we have to keep checking all that for what is linked...etc.