Figure the handle (Psub) is electrically noisy and hard to
quantify / predict due to many return-current loops. A
DNW can be used to return Ptub body noise currents to
a quiet ground, equipotential but without the activity,
instead. Provided you have a sensible impedance from
the DNW to "wherever". Especially if there is a lot of digital
in the mix, putting the analog / RF / jitter-sensitive digital
in a "clean pocket" can pay of in detail spec performance.