Puck.JS PCB design question, why a separate blue PCB from Raytao Corporation, what about it??

I don't mean to speak for Gordon here, but regarding modules like that in general - It is common practice to use modules like that when building boards that include microcontrollers with builtin RF functionality or RF communication ICs, particularly for products being made in relatively small quantities. For example, the ESP8266, ESP32, RFM69, and many others are also normally done that way. By taking advantage of the expertise of companies specializing in RF design and using these standardized modules, designers can reduce costs and implementation risk while streamlining the design process. Those raytac modules are used in many products, not just the puck.js - it's an off-the-shelf module.