The purpose of posting these photos here was to show that attenuators are sometimes neede for a test setup. I did not mention the actual purpose of inserting attenuators between antenna and ProStick to avoid confusion, as it did not relate to tests using a VNA.
However as you have asked for an explanation, here is the post where full detail of why the attenuators were inserted in antenna circuit is given. It was meant to show that ProSticPlus gets saturated without an external filter, and performance drops to near zero, but if attenuaters are added, these dampen the interfering signals and performance shoots up even without external filter.
https://discussions.flightaware.com/t/announcing-the-pro-stick-plus/19202/125