Hi A320drvr,
Is it to prevent any inadvertent tow bar movement
The NWS lock out pin prevents any HYD powered nose steering input fighting the tow bar. The PTU won't run in this case because you don't need it.
If you look at FCOM, DSC-29-10-20 Generation, PTU
PTU.png
you'll see a clever bit of logic to control when the PTU will run automatically and inhibit it when not needed.
The PTU will run during Single Engine taxi (L/G shock absorber compressed) and with the parking brake off. It doesn't run when stationary with the parking brake on. (Because you don't need it).