You're right, signature exposure will only be visible to those who haven't changed the display options. Some users may not want to see signature and avatars for personal reasons, but not many disable them.
Forum members can disable avatar and signature for two reasons:
They see avatars and signatures are very annoying so they turn those things off. It's their personal favorite on how they use the forum and see other forum members and posts without avatars and signatures.
They value their privacy and don't want other people around see their accounts with avatar, signature as well as avatars and signatures of other forum members in posts. This second reason is only about privacy.
Yes, the more people are exposed to a brand, the more curiosity it will generate. This will certainly lead them to visit the website, in this case the casino, to learn more and, depending on the budget, even try it out. The length of time spent on the forum is a way to build trust with users; it's unlikely you'll want to deposit money on a newly created website. I also believe that general campaigns like, signatures, social media, and articles, contribute to increasing brand visibility.
More exposure means more chances of getting an efficient marketing program and bringing some postiive results back to the company after spending money for marekting in signature campaign. To trace its effects, there is ref link in signature that belongs to the casino company or the signature campaign manager for getting data and assess how a campaign brings new registered users to the company.