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.