JITX's has modularity and reusability at its core. By encapsulating subsystems within modules, EEs can reuse subcircuits across their design, or across multiple projects. This leads to a boost in productivity, as EEs no longer need to spend valuable time redesigning systems. Any updates to a module can be easily propagated to all instances where it is utilized, ensuring that designs always benefit from the latest enhancements.
The USB-C Cable Tester JITX Recipe has an example of modular design for reusability. We want a reusable circuit generator that, given a color, selects the proper LED and creates a ballast resistor to get the desired brightness. We define this once and now all we have to do is change the color argument, and we will instantiate the correct LED - no need to keep loading datasheets and searching suppliers, we just do that once, turn it into code, and reuse the same component with no extra effort.