I will say that I know of a few locations from my travels that I think use QR codes specifically for entrance. La Sagrada Familia in Barcelona; The Book of Kells in Dublin; and any Premier League football (soccer) match. And I think The Churchill War Rooms in London sell tickets at the location, but I think almost everyone pre-books tickets online. I know we did, when we were in London, a couple of days in advance with a smartphone.
That said: you can, of course, purchase the tickets before you leave home and print the QR codes on paper. (Obviously that’s the case for Premier League games; I sincerely doubt there are day of game tickets available.)
But it sure came in handy to have all of those tickets on a smartphone, if only for backup purposes (you lost the paper, for example, or the QR code that you printed was not readable.)
Apple Pay using a smartphone was also a bit more convenient (for me) than taking a credit card out of my wallet and tapping or inserting into a machine. Our last two-week trip to the UK and Ireland we barely used cash at all - just for taxis, and I think even then we could have managed with credit card or even Apple Pay, though I think the taxi drivers really preferred cash. I had to convert both Euro and Pounds when I returned home - we had way too much.
Plus, as others have said, it comes in really handy to have a GPS-equipped phone if you get lost while you are walking about - just to find out your actual location. More than once I’ve stopped and carefully how to get where we wanted to go. Using an Apple Watch with walking directions going on Apple Maps makes it so you can follow directions and get directions in your wrist so you don’t have to worry about someone snatching the phone from your hand.
I’m at an age now where I want to make things easier on myself, not harder, when I’m in an unfamiliar location. Using paper maps is surely possible, but so much harder.
Of course, all of that is possible with a “burner” Android or iPhone and a little preparation at home before the trip. Not so much with an “old-fashioned” feature phone.