After all, aren't you already playing with their system? In this case, it seems like the card's function doesn't really matter. I don't think it serves any purpose other than tracking your balance and incoming and outgoing transactions, and it also has to do with the database. There are also tiers for these cards, with a more established member receiving a different card, and a more reputable customer receiving a different card.
I don't understand why.
This makes sense, and this card system is just an option, so it will be up to you if you avail it or not. So for me, there is no downside on customer's side, especially if it's free and less hassle to avail, like other card systems of grocery, restaurants, gas stations, where even it's free, you don't still want it because it's very hassle to process everything you will get it.