To expand on what the posters mentioned: HN is an international community, and the USA is an outlier in having an ample choice of banks. Many countries only have 3–5 usable banks, and all of them require a phone app for 2FA authentication in order to log into their website. Moreover, it happens that banks remove certain functionality from their websites, so it can only be done through the app.

And beyond that, in some countries the strong authentication for logging into government services has been implemented through banks, and so one cannot visit one’s local tax-authority or healthcare portal without a phone that runs the bank app.

Lots of HN readers located in countries outside of the US would find your answer infeasible (even the bank website requires their app for authentication, and this is the case for every bank).

website says to authenticate with app, now what?