Earlier quoted context omitted.
Why would you ever use a EC card instead of a credit card? Credit card companies literally pay you to use their cards. Why turn down that money? I carry two credit cards because one is American Express and pays me more but fewer places accept it, so I have a backup Visa that pays me a bit less but everywhere accepts it.
("EC card" implies Germany.) The EU imposes a maximum 0.3% fee to the merchant for accepting a credit card. This is to avoid any unfair competition between cash and cards, and between poorer people not eligible for cards that get cashback. The UK used to have cards giving 2-3% cashback, sometimes more, but they've all been withdrawn. I don't bother with a credit card, since it's one less automatic bill payment and on…
https://www.americanexpress.com/uk/content/platinum-cashback...
I have a card that gets me miles instead of cash. I did the maths and worked out those were better value for what I wanted. Again, they're just free miles and I'm always flying with the airline anyway so I'm never making extra trips in order to spend them.
The genius of the system I use is that I have multiple credit cards (American Express, Visa) but actually they all just feed into the same account with the same bill to make managing them simple.