Maybe not for you, but you don't know his bank. Obviously I can't pay with my phone if I sit at my computer and use online banking in browser, so obviously payment with phone app won't work if I use browser on phone, for all other online banking functionality there is no benefit in the app, all of my 3 banks I regularly use don't require any card reader or even smartphone, all I need is any phone being able to receive SMS with 2FA codes, if my bank required app to use online banking I would just stop using their services. I am not going to bother with app in the phone while sitting at computer with 32" display and proper keyboard.
Not all banks. Monzo doesn't have banking through the website, and I doubt they're the only ones.
Most European banks either don't have a web banking at all, or require a mobile app to log into it. There are exceptions but they are rare and becoming even rarer.
There are app only banks, you make your choice and you pay your price.
I keep seeing people say this, and it keeps being true that you can just go to your bank's website and do whatever it is you want to do.