In LTE, reordering is at RLC while in NR, it is at PDCP layer.
What is the advantage of NR vs LTE?
AFAIK, it helps, amongst other things, to alleviate processing demands, therefore allowing higher power savings.
I think this is done to support the split bearer mechanism. Be it any DC split, there is only one PDCP entity as the split happens after PDCP (into multiple RLC entities). So, it makes more sense to do the reordering at PDCP level at the receiving end and then deliver the SDUs to IP layer.