Maybe they get BTC (or crypto) cashbacks. 

Maybe for tax reasons too: if you spend fiat from an overcollateralized Bitcoin credit line, it’s not a taxable event (as far as I understand).

This way, you can “live off bitcoin” without incurring a capital gains event.

teemupleb

I don't see the need to use a BTC credit or debit card, what is the difference then from using fiat money? I prefer to make payments in Lightning or even with the main Bitcoin chain, even if there are not so many options.