Theres no use for NG+ beyond stuff like this. Theres nothing extra.
There is definitely a use. Geralt is the most renowned and probably the most successful Witcher known across the land. It was immersion breaking for me that he was a low level, unskilled Witcher with terrible armor and weapons along with no knowledge of the best Witcher alchemy at the start of TW3. I understand this is a game mechanic in order to incentivize progression but in the end playing NG+ with all of the skills, equipment, and potions I have earned equipped will feel like a much more proper start to the game as I undertake a replay.
On the topic of the Gwent cards I see both sides of the argument. These are cards that you have earned in the game. However several of them are also subplot rewards and you would probably not undertake playing the same people again if you already had the cards. Admittedly I am speaking from inexperience with Gwent as I do not enjoy it but to me a Gwent quest reward being stripped is the same as not starting with Kiera's lamp. Quest/plot device item.
EDIT: I do think that you should retain any purchasable Gwent card that you obtained directly from a vendor for Crowns and that is probably the appropriate compromise here.
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