I believe signature campaigns definitely bring customers to different gambling sites. As we can observe in the service section of this forum, there are a lot of signature campaigns which are hosted by several gambling sites. They will not do this in the first place if they are getting nothing from it. Signature campaigns act as an advertisement for them just like tv commercials. People tend to notice more about a certain site if they are always seeing it in different threads here in Bitcointalk.
Well said it is one form of promoting their sites and specially they choose the great design to catch the eyes of many users here and they click the site promoting by each users, more users more potential income in gambling sites. That is why 1 gambling sites starts in signature campaign to gather more playing in their sites.
Not just for gambling sites. As you can see, there are lots of businesses who runs a Signature Campaigns. There are Bitcoin Exchange Sites, New Altcoins, bitcoin related businesses and other who runs Signature Campaign. This makes their sales to increase because more and more people are knowing their site because of that campaign as an advertisement.
I believe running a Signature Campaign here in the forum is much cheaper than advertising in google adsense or other advertisement company that's why most of the businesses are choosing to run a campaign instead.