I've just done 3 plane trips in the past 6 months and ... resistance is futile. You need a phone, and it needs to be modern and totally up to date with operating system and security updates. If for nothing else, it's now the only way to fly. You almost can't survive without your airline's app. They will ping you constantly with reminders and updates and, most importantly, the boarding pass. I'm still old fashioned enough that I prefer a paper pass and when I'm near a computer and printer, that isn't hard. On the road, it's hard. And once you check your bag, they'll send you a confirmation and the claim check number. Want to change or update your seat? Hello app! Want to double check your itinerary, or get notices that one or more of your flights are delayed or cancelled? That is how they will reach you. They might also send an email but again, how do you read email when on the road? Phone!
Also on United, you can no longer pay for stuff onboard without the app. Not just cash, now they won't take credit cards either! Your card of choice needs to be preloaded into their app (before you board obviously) and it is the only form of payment they will accept. And I was watching while sitting at the United club, 98% of the guests had their boarding passes on their phones. Either by necessity or choice, but that's just the way things are.
In Europe, want a menu at the restaurant? You'll often have to scan the QR code. Want a self-guided tour at a museum or site? Often you scan that pesky QR code again for that. And if we go back to COVID passes, which is not impossible, guess how they will send you the certificate?