A merchant is paying a cost for the transaction -- whether it be credit card, debit card or cash. Credit and debit card they pay the merchant payment card fees (not sure for a gas station but many merchants pay in the range of 3% for credit card transactions, though less for debit card). For cash, merchants need to maintain an inventory of cash (cash management), and that may include courier costs, or even just a trip to the bank by staff.

With cash, there's the risk of ending up with counterfeit currency. With credit and debit cards there are disputes, and the risk of chargebacks even.

A merchant accepting bitcoin gives the merchant a No-KYC path (which, if that merchant were to want to do other ways, would incur a premium). But even if the merchant doesn't care about the No-KYC bitcoin, but does want to acquire bitcoin (e.g., to pay vendors, staff/payroll, or themselves as income, they would pay a fee to acquire bitcoin on exchanges.

And finally, a merchant who accepts bitcoin but then needs fiat has an opportunity to sell that bitcoin at a premium -- whether that is done to a local repeat buyer or via P2P trading platform. Some merchants essentially turn around and sell the bitcoin to their customers (less so in western countries, but this is not uncommon where the regulations are more lax or unenforced).

But as justification, simply giving an incentive for customers to pay in bitcoin is something to help bitcoin gain traction. In many situations, doing that opens the opportunity for a conversation about bitcoin, or even helps bitcoin's image. Someone might have been on the fence about bitcoin, but then seeing they can pay using bitcoin at the market might help them feel more confident about buying bitcoin (knowing there is a place they can spend it if they want to sell/cash-out later).