what is the purpose of having a "thru cal" on RF PCB?
With the thru line you can do a simple response calibration.

If you measure the response of the circuit including the chip, and compare it with the response of the thru line, you can get a good idea of what the performance of the chip itself is, without the effect of the connectors and transmission lines you used to connect to it. 

This kind of calibration isn't as accurate as a SOLT or TRL calibration, but it is better (if you want to know the response of the chip itself) than just assuming the connectors and transmission lines are perfect and lossless.