I loaded my Aussie dollar then converted it to Thai Baht and the card will automatically use the local currency first.
I only used grab for food and transport with the digital wise card which is linked to the one wise account. Meant if I was unfortunate enough to perhaps get hacked, I still had the physical card. It was very easy to set up a grab account too.
When I was in Thailand recently I used my Wise card whenever I could (supermarket, some restaurants, activities). There’s usually a small charge to cover the merchant fee.
I used my Wise card to withdraw from ATMs but I had already loaded the card with Thai baht. aTMs do charge a fee so I used to withdraw a significant amount to limit the number of times I needed to get cash.
The two numbers is for your security. In the event you misplace your physical card, the digital one can still be used as it’s a different number and vice versa. I used the digital number when I set up my grab account.