I personally don't really matter the rank of a bounty manager, but the reason I often participate in bounty campaigns from managers as you mentioned is that I find something convincing from the projects they are working on. When a bounty manager like Hhampuzs handles a project we will be curious and want to know what the advantages of the project are so that he wants to handle the project. Automatically we will find out about the project, and if we find something convincing there, then we will participate in their bounty campaign for that reason.