As @Canadianspruce said, with Commercial cards the card issuer earns more in the interchange fee, see: https://en.wikipedia.org/wiki/Interchange_fee
The cap used to be much higher in the EU (or was there even any cap at all?) but then the EU ruled that the card issuers charge too much which results in lower acceptance rate cause less merchants are likely to accept “expensive” cards and rather stay with cards like Maestro or ELV (SEPA), which is only 0.19 EUR / transaction for most merchants (at least in Germany) and no % whatsoever.
So in the EU for personal cards its: 0.3% for credit and 0.2% for debit cards.
AmEx and business / commercial cards are excempt!
Thats why when the EU introduced the law many great cards vanished, cause it was one of the main incomes for many banks. Suddenly that income went away, so they cancelled lots of miles and cashback cards.
But don’t confuse interchange fee with merchant fee! Example:
You go and buy something from a Nike store:
Nike pays a merchant fee to their bank / payment processer, such as WireCard, First Data, …
That fee can be anything from 0.5% to 5%, depending on your revenue with the payment processor. For small businesses around 2% is the normal, so thats what the merchant (the shop) has to pay for accepting cards.
WireCard then contacts the customers bank (lets say you have a card with Barclays)
If you have a personal debit card, WireCard can only be charged 0.2% from Barclays, with credit 0.3%
If you have a commercial / business card, they can be charged a MUCH higher fee, so higher income for Barclays.
Thats why every bank loves and prefers to give you out commercial / business cards since they make much more money from them than personal cards. I believe this is the reason why Curve started with those as well.
BUT banks are required by law to ask you if its really just for commercial expenses.
Some banks just let you make check a box and say, hey, we asked, and he said yes, even if he uses it for personal expenses or doesnt own any business whatsoever, thats not our fault.
Some banks really ask for proof of ownership of a business.
As a customer it doesn’t really matter:
If you want to support small businesses, be nice and use your personal card cause its cheaper for them.
If you want to support your bank, use your commercial card cause they make much more money