why PRM?
PRM increases the landing rate during peak periods affected by low cloud, to a number more-or-less equivalent to IVAs.
It reduces overall holding (but you get an extra 20 track miles for free).
It avoids the situation where a PA31 backs up the whole sequence to minimum approach speed from 15NM by allowing overtaking during IMC.
It gives a couple of controllers a couple of hundred bucks each for coming in on their day off for the privilidge of staffing PRM.
From what I'm told, it's a inefficient traffic management tool however the "saving to industry" is enough to warrent continued use.