Id say for discounts is the only reason why they would choose to do so.
In order to make a payment to someone, there are a lot of middlemen who earn interest. In the case of bitcoin is not the case. But in bitcoin can make a waiver or change the terms of payment. This is also an important aspect.
So the customer who pay with the bitcoin should pay lower price.
Why would I sell my goods for a lower price for BTC when I can get a better price using Fiat?
Why would customers use bitcoin? Only if they can earn rewards or some shit like that. Maybe another alt coin such as GoCoin or something for the rewards..
I don't see any reason for customers to use crypto over fiat at this time or in the near future. Bitcoin remains a speculative instrument and nothing more.