It is always wise to have a small amount of the local currency. You don't know until you do! Like others have said, street vendors, small shops in villages, and restrooms are the most significant reasons.
I'm bringing some other foreign currency from countries I doubt I'll returned to. I will bring that if I need to use a currency exchange since the bills just I laying around in my desk have little value.
I have traveled extensively and learned this lesson. In New Zealand, which claims to be cashless, I needed coins for a locker at the entry of a museum. Fortunately, the receptionist told me the lockers on the second floor were free. Problem solved.