Firstly don't think only the high ranked members are getting the best from the signature privilege, everybody is, the only reason they are been paid more is because the forum has a ranking system and as such it's quite normal for those in the higher ranks to earn more due to the nature of their signature space.

With Senior member and above rank, your signature ads becomes more visible. The high ranks users most have spent more time on the forum and with the introduction of the merit system, they must have earn their way to the top through their quality contributions to the forum.

For how signature works, after projects has picked interest in launching a campaign on the forum either managed by themselves or a campaign manager from they forum, hired promoters are required to wear the signature code and avatar of the project and continue engaging in conversation on the forum.