Bitcointalk signatures provide a form of advertisement and exposure for different projects and services among the community members and the guests that visit the forum from time to time.
So If you project or service is of interest, people will get to know about it and access it through the signature links. I think the reason this kind of advertisement is common around the forum is because;
1. It cheaper compared to other forms of advertisments
2. Targets the right audience. If you service is crypto related, there's no better place to advertise than this forum
1. Not that cheaper because if we do consider the amount been spent on weekly basis then i can say that it is still a significant amount or budget.
2. This is right! When it comes to targeting the right audience then this would be the best place for that considering that this is the main bitcoin forum, then its no brainer that you would really consider this first.
Although results arent that guaranteed but we know that it is really worth on investing or allocating budget for marketing on this forum rather than on going into there mediums
but if there still budget then the more the exposure the more the better.