NSS is a driver included in the OEM firmware that allows for "NSS cores" to do work that the stock OW build does on the CPU if I am not mistaken. (See the OEM boot log here).
Why use it? If you suffer from buffer bloat as I do, and if you're buying bandwidth >200 Mbps or so, you will be unable to manage the bufferbloat without suffering a speed cap due to CPU saturation on the R7800.
With OW patched to use NSS in the thread I linked above, and using these settings, I get 650+ Mbps downloads that are very low latency in bufferbloat tests. Without the NSS patch, the best I can do using SQM on wired devices is ~220 Mbps. As well, devices connected to my WiFI with NSS patched OW can see d/l speeds 400-500 Mbps. They too were capped to 200 Mbps before it.