I am guessing that you do not hold an Amex card. I have found it...

It is mostly an additional payment option which might help to capture sales from those who prefer to pay in increments and would otherwise only be browsing. I suppose it depends on your target market. e.g. It may not suit a $2 shop, but it seems to be working for Lovisa.

I am guessing that you do not hold an Amex card. I have found it accepted in many places, from super markets, department stores, restaurants, to small cafes and hotels. A quick look at the Amex website will show you that major retailers do accept it.
https://www.americanexpress.com/au/credit-cards/about-credit-cards/acceptance/shopping/

They also provide a map of the smaller retailers, which are plentiful in Australia.