[atlas] Private Probes: What's the Point

Hi All, I've run into a few probes marked private, and when I've found & asked the owners, they didn't realise they'd done anything restrictive or harmful to researchers by marking their probes private. Intrigued at this phenomena, I had a look at the probe metadata. Right now I think there are 1,534 Atlas probes that are active and connected to the network, but marked private. What's the point of having private probes in the network? Do their hosts still earn credits they can use on public probes? Should they be? Are private probe hosts helping the project? The FAQ says nothing about private probes, so I thought I would ask here. Thanks, Jon