How to accurately determine Navigation Accuracy from FAA's PBN Criteria (Order 8260.58A)?
Im more familiar with EASA then FAA but im sure there are similarities: 

why a desired ground track would require a smaller navigation accuracy value?

RNP 2: the aircraft must be within ± 2 nm of the indicated position 95% of the flying time (Required for Basic RNAV).

RNP 1: the aircraft must be within ± 1 nm of the indicated position 95% of the flying time (Required for Precision RNAV).

So flying in a congested airspace (for example the NAT-OTS routes) ATC can accommodate more aircraft with smaller separations.

the goal is to provide an adequate Obstacle Clearance Area

Yes, as well as separation and spacing of aircraft. 

doesn't this mean that smaller accuracy value would lead to a smaller obstacle clearance area?

In theory yes. But Obstacle clearance requirements around Airports and high terrain take into account all aircrafts including general aviation and older aircraft without PBN, RNP equipment so while you are right with more accuracy the approach and departure corridors lead to smaller clearance areas due to the regulations the actual clearance areas could stay the same. 

More likely there will be changes at the approach and departure procedures (points, fixes and routes). 

Hopefully you found something helpful.