I've been to a few searches where there are police vans, stations and volunteer tip tents. I'm certain another WSer will have a better explanation, but they are a group of volunteers who are usually close to a victim of a crime, misadventure or accident, who organize to highlight information about the victim and the circumstances, and hope to draw possible tips from the public to aid the investigation
They seem to be less intimidating than making a formal tip to the police, especially when the information you have might seem insignificant.
No one is searching harder than the victim's loved ones, and if you have a tip, you might be inclined to want to tell them directly, fearing it might get lost in the flood of information that the police have to sift through. (Tip tents tend to be manned by people close to the victim, or investigation, at least the ones that I have seen during searches).
They are also a place where you can go offer support to the victim's loved ones. Setting up a tip tent also draws attention to the search or crime, and might help get some much-needed media coverage.