This is primarily done to prevent low-frequency common impedance coupling (ground loops) and provide increased capacitive decoupling at higher frequencies.
This reduces common impedance coupling and prevents the equipment and subsystem boxes and cable shields from acting like antennas.
For every routed signal, we have to ensure that we have a proper return path that is located where we want it and that we don't obstruct already created paths, because signals sharing a part of the return path have common impedance coupling (energy coupled from one signal to the other).
We need to establish where the path is and that it is not shared for two sensitive signals to avoid common impedance coupling between them.
The LFM solid copper slug has significantly lower ground return inductance that reduces the common impedance coupling
within the module.