Ease of cleaning.
Cycles smoother.
Quieter.
Good things are that its easier to clean off powder residue which has a tendency to embed itself on regular phosphate. I think the bolt to carrier being both NiB also makes it more reliable. Same with the carrier being NiB, more lubricity for better words, against the upper receiver. Still requires lubrication as everything else, but since you can buy for a little more money, I've been switching all my guns to NiB carriers, but nice having both NiB carrier and bolt.