If I remember correctly, the main difference is that you would have to manually sell into the BitPay card, which just spends fiat at that point.
It's like selling BTC into your bank account and then using that EUR balance with your traditional debit card.
Instead, with wavecard: 1. You hold BTC until the last possible second before paying at the shop (no manual pre-loading) 2. It connects to your Lightning wallet via NWC that allows for automatic topups to replenish the card with more BTC after each payment.