ICAO Doc 9613 (PBN Manual)
Part A - Chapter 3
3.2.2 - Separation minima and route spacing can generally be described as being a function of three factors:
navigation performance, aircraft’s exposure to risk and the mitigation measures which are available to reduce risk — see Figure I-A-3-2. Aircraft-to-aircraft separation and ATS route spacing are not exactly the same. As such, the degree of complexity of the “equation” depicted graphically in Figures I-A-3-2 and I-A-3-3 depends on whether separation between two aircraft or route spacing criteria is being determined.

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This extract is taken from a relatively niche section of the PBN manual, but does dictate the concept well. If navigational performance can be improved, ATS routes could be placed closer together, allowing a higher traffic density through the airspace, more fuel efficient routes, etc. This concept is present throughout the whole PBN manual, and is one of the driving factors behing PBN as a whole.