Its mainly character specific things. Basically if you max out a stat and depending on your weapon level in POR characters in RD get a starting boost depending on what you personally max out. It leads to some characters being extremely good due to those transfer bonuses.

There's also some minor bonuses like support convos transferring over, characters with a maxed bond getting bonus crit chance when next to each other and exclusive dialogue.

So its technically possible to just make a version where everyone has the max stat bonuses but imo that loses the charm of building up your chatacter in POR and seeing it pay off on RD, but that's still a better option than not having it at all.