NUC is used as a substitute for the US $. All tickets are priced in NUC's but then converted at that day's BSR (base rate) into whichever currency you purchase your ticket in.

2 other reasons for the NUC are:

1. some countries restrict foreign currency transactions by its citizens so a NUC can be a good way to price in the local currency w/o actually fixing a rate. (good for countries where the currency fluctuates often).

2. the NUC also allows you to purchase tickets originating outside of the country of issue.