Out of curiosity, what in your context means "Signature Campaign" ? I would also go with Bonuses and competitions being big driver and contributor overall.
As simple as you called/viewed the Signature Campaign here, it is so powerful to deliver more for companies even as many are wasting their money for marketing purposes elsewhere. With a huge number of active users and readers here, there's no doubt that this forum is a huge marketplace where buyers and sellers are being brought together regularly. This may not be a direct engagement, but really, it delivers with the track traffic and converting records.
Besides, I do not blame the OP on this, and as he said, it is out of curiosity since the Signature Campaign style of marketing is not so popular, unlike the normal ads/banners, influencers on social media platforms and adverts on websites. But really, this is a huge forum that has more traffic than most of the websites that companies use for adverts, and many of the influencers on social media platforms do not have the high number of readers and SEOs that this forum generates.
And of course, those bonuses and competitions you mentioned are happening here, many would add it as a point of attraction with the design and writing on their Signature Campaigns to easily attract the target audience. For this, I am sure that brand new casinos can still come here straight away to advertise and get popular. However, one thing they need to put in place to help us with for better success is to have a good platform and give quality services that are worth making the needed conversion.