Sometimes I don’t want to walk around with my valuables on me, I just bring a small sum enough to satisfy my groceries.
I do this especially when it’s dark outside and I have to go out to shop, this way, I don’t lose anything significant if I get robbed.
Another good reason is that some shops have to pay a fee for being able to accept digital transactions, for some shops, the fee is quite a lot.
So paying with cash is preferred and helps them a bit.
Lastly, while it sounds shady, being able to pay in cash keeps me from being traced by the government. I don’t really have anything to hide but having the option to be anonymous If I want feels good.
Cash is useful in cases where your phone/card isn't working. E.g. if your phone has run out of battery, you are in a location that has a poor signal, your account has been locked due to a suspicious/unusual transaction (even if legitimate), etc.
You don't need it all the time, just for emergencies.
Systems you want to use in an emergency have a tendency to start failing when you don't exercise them regularly. That's my primary reason for using cash. Privacy is the second. The tactile sense for how much money I spend is the third.